A user is anyone on your Slack who gives a token in appreciation. So if Alice wants to thank Bob for some act of helpfulness, she gives Bob a token in Slack, and that adds 1 billable user for that month. Billable users are calculated each month, so if Alice goes on leave the next month and doesn't give out any tokens, she will not be counted as a user for that month.
A person who receives a token but doesn't give out any tokens themselves doesn't count as a user, and so doesn't get billed.
When your free trial runs out, you will need to add a payment method to your account to continue using ThanksChat, or ThanksChat will stop working on your Slack instance.
We do not ask for card details when you sign up for the free trial, so you will not be charged if your free trial runs out, you just won't be able to use ThanksChat again until you have added a payment method.