The best place to get started with learning about a given Linux/Bash command is to reference the manual page or manpage of the given command.
Here is a link to a top manpage. In shell, you should be able to read the manpage by simply executing man top. I will also include a link to a blog explaining top.
The relevant part to your question can be found at section 2b. TASK and CPU States of the manpage:
As a default, percentages for these individual categories are
displayed. Where two labels are shown below, those for more recent
kernel versions are shown first.
us, user : time running un-niced user processes
sy, system : time running kernel processes
ni, nice : time running niced user processes
id, idle : time spent in the kernel idle handler
wa, IO-wait : time waiting for I/O completion
hi : time spent servicing hardware interrupts
si : time spent servicing software interrupts
us and ni are the percentage of CPU usage spent on un-niced and niced processes respectively. Nice values are user space processes that are either nice or not in that they can be given a priority value that either cooperates and gets out of the way of more important kernel or system processes or does not. Here is a link to a fairly straightforward explanation of niceness and priority.
The others should be rather straightforward:
idle is how much of the processor's capacity is idle or unused. io is the Input/Output queue of the processor. irq and srq are hardware and software interrupts respectively.
If you want more information on how to sort top output, here is a relevant Stack Overflow post. Additionally if you want to know more about clearing cached memory/buffers, here is a U&L stack exchange post.
Please read over all the links I have provided and if needed you should dig a little deeper and research more into how Linux processing and memory handling works. There is a wealth of information out there online.
