On the free plan, you only get contact insights on a contact’s website activity for 7 days. If you were to upgrade to starter, it would allow you to see the insights indefinitely. For example:
Prospect comes from social lands on page xyz
Prospect visits page 123
Prospect completes form on page 123
On free, you will have those insights for a week, on starter, forever.
Note, any contact properties (or other objects) are stored indefinitely on free. This is only related to activity.