Understanding the activity monitor of mac

• You see eight cores because your Mac has an Intel chip that creates virtual cores. Your hardware only has four.
• Swap used refers to the amount of memory that has been "swapped" to the disk for temporary archival. This happens when your system runs low on memory, but unless your "memory pressure" is consistently high this isn't indicative of a problem.
• Because of Swap, there is no requirement that the amount of memory used is less than Physical memory.
• I'm not sure there's an answer here. This graph doesn't have units, it isn't intended to be used except to estimate changes in usage. I would recommend looking at "Data read/sec" and "Data written/sec" and comparing these figures to your SSDs rated performance (you can google what your computer should be getting). In general though you will not get that figure because unless you're writing/reading one large file there's a lot of additional processing and overhead.
• I/O refers to your disk read/writes, and Data refers to the amount of data being read/written.

The other answer from @Gilby has more detail - please refer to that as well!

I have taken your questions as relating to four areas which I will attempt to answer in the same sequence. I am writing this because I feel that @Ezikiel Elin answer is, in some parts, misleading.

Physical cores and hyper-threading

Your CPU has 4 cores, but also has Hyper-threading which presents 8 virtual cores to the operating system. This has the benefit of allowing the cores to more rapidly change from running one process thread to another thread (quite likely from a different process). There are circumstances where this can slow throughput, but it is usually beneficial.

Activity monitor presents all 8 virtual cores. The first physical core is shown as the first two virtual cores in the graphs. The second physical is in graphs 3 and 4, and so on. In your example, the cores are only lightly loaded and not sufficient to make significant use of hyper-threading.

Swap and Compressed

There are time when macOS and your running applications need more memory can than comfortably fit in physical RAM. In that case the memory management of macOS frees up RAM in two ways:

1. Take some memory and compress it. In your example, 906 MB of physical RAM is being used to store some unspecified (but probably about twice that) of macOS and application memory. Of course, the memory will need to be decompressed before it is next used.

2. Move some portions of memory out of physical RAM and store them on the boot disk. In your case, 6.64 GB of disk space in being used in this way. When an application requires to use that memory in will have to be brought back into physical RAM. This moving portions of memory out and in to RAM is called swapping and the disk space used is in one or more swap files.

Compressing and decompressing is a relatively fast action using a highly efficient algorithm. Swapping is slower. When boot disks were hard disks this was a key cause of slow system response. With SSDs, this is quicker and leads to a more responsive feel to the system.

macOS's memory tuning has a preference for compression rather than swapping though in your example it has decided that rather a lot of memory must be taken out of physical RAM and placed in the the swap files. Ideally the memory tuning will have chosen application memory that is unlikely to be wanted for some time.

Memory calculations

Memory allocations can be counted in more than one way, but as presented by Activity Monitor:

App Memory + Wired memory + Compressed = Memory Used

Wired memory is memory which can't be compressed or swapped. I don't like the use of "memory used" in this context because in reality nearly all the physical RAM is usefully occupied. But that is what Activity Monitor calls it.

Memory Used + Cached Files + Free memory = Physical Memory

So it is true that Memory Used + Cached Files is always less than or equal to Physical RAM.

With a simple calculation you can work out that your Free Memory is only 0.10 GB.

Your "memory pressure" graph is green which indicates that macOS is able to manage memory with few bottlenecks.

Disk IO and Data

To the left of the graph, Activity Monitor shows that number of read and write operations (called IO in the graphs) both as totals since boot and current rates.

Each disk operation can read or write variable amounts of data. The values for amount of data and current rates are on the right hand side. The graph calls this DATA (which seems reasonable to me).

From your graphs and current counters, the Mac looks fairly busy from the disk point of view. But the graphs have no units which does limit their usefulness.