A prat fall (sometimes called tripping or slipping) is a minor game mechanic in Super Smash Bros. Clash, which kind of returns from Super Smash Bros. Brawl. This is when a character, under circumstances, falls on their feet and sits down, looking confused. You have about 8 invincibility frames in this state. How many times each character and registered name has suffered a prat fall is kept track of in the VS. Records.
Random prat falling has been removed from Clash. Fans are happy. Banana Peels, Luigi's Negative Zone Final Smash and attacks that can cause these all still have chances of causing prat falls. You can still prat fall randomly on certain terrain, such as on ice, but it's been lowered from Brawl.
A prat fall is also considered a knockdown and as such you can hit OTG after landing a prat fall. Most options available after being knocked down are available after a prat fall, although you do get a different get-up attack that has way more invincibility frames than a knockdown get-up attack.