This is a feature of your anti virus programme which keeps a record of average usage and then warns the user if this increases.
This can happen because you have a lot of tabs open at the same time. It can also be because you have a lot of add-ons. It maybe because you have downloaded large images. When you get this message try closing down any browsers - this should help.
It is not necessarily anything to worry about - just your anti virus doing it's job. It is normal for some programmes, particularly browsers, to occasionally require more system resources.
Only if a programme continually continues to demand a very high percentage of system resources over a prolonged period of time should you be worried.