What is porn?

My little brothers (2,3,4) wanted my phone to play a game. I deleted everything I could, but then I remembered YT. I love to watch the occasional *ahem* video to help me sleep. So I made a throwaway account and logged into it on my phone. Then I gave it to my bros. One of them decided to watch videos, and then he asked why there was a naked man on the screen humping something… he’s now in his room with no TV, tablet, or phone for the rest of the night. So what do I tell him?