An activity is a trackable action committed by a visitor to your website.
Typically, these include pages viewed, forms submitted, offsite links clicked, PDFs downloaded etc.
You can find a list of the activities of a particular visitor towards the bottom of their contact profile.
You can filter activities on an individuals profile by clicking the filter button and customising it specifically to what information you need. For example if you would like to see what activities they had done on your website on a certain date.