All of the programs running on your computer share CPU (processor) time and memory. If you run many programs at once, or if some programs are doing a lot of work, a higher percentage of both of these things will be used.
If the CPU usage is around 100%, this means that your computer is trying to do more work than it has the capacity for. This is usually OK, but it means that programs may slow down a little. Computers tend to use close to 100% of the CPU when they are doing computationally-intensive things like running games.
If the processor is running at 100% for a long time, this could make your computer annoyingly slow. In this case, you should <link xref="process-identify-hog">find out which program is using up so much CPU time</link>.
If the memory usage is close to 100%, this can slow things down a lot. This is because the computer will then try to use your hard disk as a temporary memory store, called <link xref="mem-swap">swap memory</link>. Hard disks are much slower than the system memory.
The memory shown in the <gui>Resources</gui> tab is system memory (also called RAM). This is used to hold programs temporarily, while they are running on the computer. This is not the same as a hard disk or other kinds of memory, which are used to store files and programs more permanently.