ADP is an acronym that means "Average Draft Position". It refers to the place at which a player is being drafted, on average, in drafts that have already been completed. 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: If five fantasy drafts occur and Luka Doncic is selected 2nd, 4th, 5th, 6th, and 3rd, his Average Draft Position (ADP) would be 4.0.