They're basically just.... peas which are mushy haha.

https://en.wikipedia.org/wiki/Mushy_peas

In the original article I linked, they're listed among sauced. That's because it's common to either pour them over chips, or dip your chips in them. I'd consider them more of a side-dish than a dip however.

I frequently buy tinned mushy peas and eat it on toast. This isn't exactly a British thing in itself, but it seems to me quite British to do. It's a very popular British food, and we love things on toast (usually beans...). Most people who I've told about this [in the UK] don't seem to think it sounds especially weird.