"ADP" stands for Average Draft Position. Depending on your source, ADP may be calculated using data from mock drafts and/or real drafts. It's a useful tool when preparing for your upcoming drafts, as it represents a player's market value. It tells you where others are drafting a player and what it will likely cost to land that player in your draft.
For example, you may love Christian Watson heading into 2023 and think he'll finish as a top-15 fantasy WR, which would make him a late-second round or early-third round value. However, if his ADP is 65 then that indicates that others aren't as intrigued by him, and you can probably wait until the 5th or 6th round to select him, while filling other needs with top talent in the earlier rounds.