The jackpot contribution is the slice of every qualifying stake that goes into a progressive jackpot. It is part of the game's RTP, but it is paid back only to the rare player who wins the pot.
Games often list the two parts separately: a base-game RTP and a contribution to the jackpot, which add up to the total. Illustrative example: a total RTP of 94% might be made up of 91% from the base game and 3% to the jackpot. For almost everyone, the return they experience in practice is closer to the base figure.
This is the main reason jackpot slots tend to feel less generous in normal play than non-jackpot games with a similar headline RTP. Check the paytable for the breakdown on any jackpot game.