Your trackers pause. The history, the configuration and the archived answers stay exactly where they are, and measurement resumes on the next top-up, in the same series.
Credits never expire, so a balance sits there until your trackers consume it. Reading stays open whatever the date of your most recent top-up, in the app and through the API alike, and your series is kept in full: come back after a break and it picks up where it stopped.
Auto-reload keeps the balance up on its own if you want it to, on a threshold, an amount and a monthly cap you set, and you can turn it off or remove the card at any time.