A Task is counted each time your Zap successfully performs an action for you. There's some more detailed information and some examples of what counts as a task here.
As you approach your allocated Tasks per month for your account, we will send you a warning email at about 80% usage then again at 100% usage.
When you exceed your allocated Tasks per month, your Zaps will stop working. You won't be charged because no further actions will be taken until either your billing month ends or you upgrade to a higher plan with a larger task quantity.