(Deliberately a bit vague, but hopefully understandable)

There are basically two types.

Anyone can request to do a payment routed through your node, and the point of LN's privacy is that you don't know who the original requester was, nor who the final recipient was. You just see the request coming from one of your peers, and going out to another one of your peers.

If someone requests to do a payment of 100k sats and your channel has a 105k sat capacity, then until that payment either completely succeeds or completely fails, your liquidity in the channel can be locked up by that. And that can be a not trivial amount of time, during which your channel can't be used by anyone else.

Doing that maliciously is called "capacity jamming".

On the other hand "slot jamming" happens for a less obvious reason: your channel can't deal with 1000s of payments being routed through it at once, because each one needs to take up an output (see: HTLC) in the commitment transaction which shouldn't be going on-chain, but you need to be able to broadcast it if necessary. But bitcoin transactions are limited in size (and certain other things). So someone can jam by starting tons of payments at once, even if they're only tiny, like 1 sat or whatever, it can still jam your channel because you can't accept any more, during that time.