Read on Twitter
Intrusive thoughts are thoughts that randomly pop into your head, and cause you distress. Most people have them, but when you have #OCD you're more likely to attach massive significance to them. An example is that thought many people have of 'what if I threw myself on the track?' https://twitter.com/ScarfmanAndy/status/1329143353800335365
Most people can dismiss this as random brain noise, but someone wth OCD might start ruminating over it (which in itself is a mental compulsion), thinking 'oh my god why did I think that' etc. The more you don't want to think a thought, the more you think it, and a cycle starts
Intrusive thoughts are often around the things we would be most horrified if we did 'what if I kill/stab someone' etc. The problem tends to be that a person is so horrified by a thought, and it is so at odds with their values and who they are, that they can't let it go.
And don't worry about asking @ScarfmanAndy! I sometimes use terms without defining them when talking within OCD spaces but if someone isn't familiar it's absolutely right to ask what it means!