What is slippage?

Getting a slightly worse average price than the one on screen, because your own trade moves the market.

The percent you see is the price of the first share. As your stake pushes the price, each next share costs a bit more, so a big trade "slips" from 62% toward 65% while it fills. Thin market, more slippage; deep market, barely any.

Foresee shows this before you confirm, "your stake moves the odds 62% → 64%" is your slippage, stated in plain words. Real-money exchanges show the same thing as an order book you eat through. Same physics, different dashboard.