What is memory pressure, and how do I relieve it?

Solution 1:

Memory Pressure is a better indicator of the health of the virtual memory system than the traditional UNIX measure of free, inactive and active pages.

For a brush up on the previous measure, have a read at:

• Isn't Inactive memory a waste of resources?

Now, the caching of files is drawn as a good thing (green memory pressure) and you will only see the pressure raise in the main window of Activity Monitor when you have a large proportion of wired memory or the system starts swapping RAM to storage and evicting too many cache pages.

The image from your window looks perfectly fine - so I wouldn't worry about reducing the pressure. If you want to reduce the pressure, you can do so by quitting apps and then letting the system manage that memory. If your Mac can accommodate more RAM, adding RAM would reduce the pressure for the same line up of programs running and using memory.

Solution 2:

Here's what Apple says the graph means: https://support.apple.com/en-us/HT201464

• Memory Pressure: The Memory Pressure graph helps illustrate the
  availability of memory resources. The graph moves from right to left and updates at the intervals set in View > Update Frequency. The
  current state of memory resources is indicated by the color at the
  right side of the graph:
  • Green: Memory resources are available.
  • Yellow: Memory resources are still available but are being tasked by memory-management processes, such as compression.
  • Red: Memory resources are depleted, and macOS is using your startup
    drive for memory. To make more RAM available, you can quit one or
    more apps or install more RAM. This is the most important indicator
    that your Mac may need more RAM.