Slippage is the difference between the price you were quoted and the price a trade actually fills at. On fomo the slippage setting is on Auto unless you change it, and you can set your own ceiling on any token trade.
Auto and custom
Auto lets the app pick a tolerance for each trade. A custom value is the most price movement you will accept: if the price moves further than that between the quote and the fill, the trade fails instead of filling at the worse price.
The app marks Auto as recommended for most swaps and does not suggest a number.
A custom value is yours to set, and on a token with thin liquidity a higher ceiling is exactly how a trade fills further from the price you saw; the app warns you when a custom value is high enough to fill at a much worse price.
Where to find it
In the app, on the buy or sell screen, expand the details to see the Slippage row. It reads Auto or your chosen percentage, and tapping it opens Edit slippage. The row shows on trades paid with your cash balance, not on a withdrawal or an Apple Pay purchase.
In Quick Trades, the same setting is in Quick Trades settings.
On the web app, it is the Swap settings gear on the buy or sell panel.
The setting is stored on the device or browser you set it on, so a custom value in the app does not carry over to the web, or the other way round.
If a trade fills away from the quote
The quoted price is an estimate from the pool's liquidity at that moment, and other trades can land between your quote and your fill. If a fill looks wrong, contact support with the token and the time of the trade and we will check the actual execution with you.