what does mean cpu 100% usage ?

i use br500 and i use over 20 devices( just computer,smartphon,cctv, sensors,etc)

these devices do not perform sophisticated tasks.

but i saw 100% usage twice.

i believe my network is under attack( i have reasons to believe that).

is it related to cpu 100% usage? or

cpu 100% usage can be happened anytime.

could you tell me under what circumstances can cpu 100% usage occur?