When you see the acronym OTF in a text message, it stands for on the floor. This is typically used as a reaction to something that someone finds funny. For example, if you sent a friend a meme and they replied with OTF, it means they were laughing so hard that they fell on the floor.
While on the floor can be used literally, it's more often used figuratively to describe how funny something is. So, if you see OTF in a text message, know that it's likely someone's way of saying they thought whatever you sent them was hilarious.