Short answer
HubSpot's KB: log calls, emails, meetings and messages that happened outside HubSpot, with associations to multiple records. The number of activities you can log depends on the object, activity type and subscription. Bulk logging is done with an import.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Logged activities don't send anything; they record what already happened.
3. Related
Log SMS by API and exporting activities.