Short answer
FDV is the price multiplied by the maximum supply that could ever exist. When it's far above market cap, a lot of new supply may still hit the market.
Market cap counts tokens in circulation today. FDV counts every token that will or could exist, including ones that are locked, unvested or not yet minted.
For most Solana memecoins the whole supply is minted at launch and mint authority is revoked, so market cap and FDV are about the same. Where they differ a lot, ask who holds the rest and when it unlocks, because that supply can be sold.
Like market cap, FDV is a paper number. It says nothing about how much money could actually be withdrawn from the pool.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.