A credit is one answer. Roughly.
A text answer costs a set number of credits, depending on the model: the most economical costs least, the strongest costs most — you pick the model for each assistant in your panel, and the price is written beside each one (the table shows them all). A spoken answer costs a little more, whichever model wrote it, because turning speech into text and back costs the same either way. A few premium voices cost a little more still; the voice picker says how much.
So 3,000 credits is roughly 3,000 text answers or 1,500 spoken ones on the model a new assistant starts with. That is an estimate, and your panel keeps the exact count.
Credits are spent only when I answer. Reading your pages and setting me up cost nothing, and before you've picked a plan, trying me in your panel is free too — up to 50 credits a day and 300 a month.