Activities are a way to record your day-to-day fundraising actions and interactions. Activities are associated with Donors or Leads, and also (optionally) with Opportunities.
As opposed to a Task (which you need to do in the future), an activity is something that you have already done and are recording. It could be logging a phone call, making notes from a discussion, or anything relevant to your interactions with a Donor or Lead, or connected to your Opportunities.
Here's how to create a new Activity:
Select the Donor or Lead that this activity was performed for.
Give your activity a subject and describe what was done.
Select the date that the activity was performed on.
Upload any files that are relevant to the Activity.
Then, either save the Activity, or save and create a new Opportunity, Task, or Reminder, which will then be linked to this activity.