Short answer
It's the profit or loss of the typical person who copied a trader: line all copiers up from worst to best, and the median is the one in the middle.
An average can be dragged around by one huge winner. The median can't: half of copiers did better than it and half did worse. That makes it a more honest picture of what copying a trader is likely to have meant for an ordinary follower.
We calculate it from closed copied positions on-chain, after fees. A trader with a great personal record but a negative median copier P&L is someone whose followers have mostly lost money, often because they enter after the trader and pay a worse price.
It's still a backward-looking number. It tells you what happened, not what will happen. You can see it for every ranked trader on the leaderboard.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.