What is "overhead"?

It's the resources required to set up an operation. It might seem unrelated, but necessary.

It's like when you need to go somewhere, you might need a car. But, it would be a lot of overhead to get a car to drive down the street, so you might want to walk. However, the overhead would be worth it if you were going across the country.

In computer science, sometimes we use cars to go down the street because we don't have a better way, or it's not worth our time to "learn how to walk".

The meaning of the word can differ a lot with context. In general, it's resources (most often memory and CPU time) that are used, which do not contribute directly to the intended result, but are required by the technology or method that is being used. Examples:

• Protocol overhead: Ethernet frames, IP packets and TCP segments all have headers, TCP connections require handshake packets. Thus, you cannot use the entire bandwidth the hardware is capable of for your actual data. You can reduce the overhead by using larger packet sizes and UDP has a smaller header and no handshake.
• Data structure memory overhead: A linked list requires at least one pointer for each element it contains. If the elements are the same size as a pointer, this means a 50% memory overhead, whereas an array can potentially have 0% overhead.
• Method call overhead: A well-designed program is broken down into lots of short methods. But each method call requires setting up a stack frame, copying parameters and a return address. This represents CPU overhead compared to a program that does everything in a single monolithic function. Of course, the added maintainability makes it very much worth it, but in some cases, excessive method calls can have a significant performance impact.

You're tired and cant do any more work. You eat food. The energy spent looking for food, getting it and actually eating it consumes energy and is overhead!

Overhead is something wasted in order to accomplish a task. The goal is to make overhead very very small.

In computer science lets say you want to print a number, thats your task. But storing the number, the setting up the display to print it and calling routines to print it, then accessing the number from variable are all overhead.

Wikipedia has us covered:

In computer science, overhead is generally considered any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to attain a particular goal. It is a special case of engineering overhead.