stompix gave a real ELI5 explanation... i'm going to dig a little bit deeper whilst still trying to keep it as ELI5 as possible...

Each block can contain at maximum 4 Mb of transaction data. 1Mb for the "real" data and 3 Mb of "witness" data.
Eventough i haven't really read up on "ordinals", they're basically an NFT built up the bitcoin blockchain. In order for them to work, they create tons of transactions. By doing so, they create more transaction data than there is space in the bitcoin blocks.

This is why other people that are legitimately using bitcoin have to outbid each other (and those ordinal users aswell) for a place in one of the next blocks.