TL;DR: An "action" is one sync operation — one record created, one item created, or one changed field synced. Every plan includes a monthly action quota. When you hit 100%, syncs pause until your quota resets on the 1st of the month or until you upgrade.
How counting works
Each of these counts as 1 action:
A new HubSpot record creates a new item on your board
Each changed field on a HubSpot record that syncs to your board
Each changed field on a monday.com item that syncs back to HubSpot
Examples
Scenario
Actions used
A deal's amount is updated in HubSpot, syncs to your board
1
You update 3 fields on one item in monday.com, syncs to HubSpot
3 (1 per changed field)
The same deal is synced to 2 boards (2 separate syncs)
2
What does NOT count as an action
Opening the app on your board
Changing your sync configuration (field mapping, direction, conditions)