If you’re saying that you literally don’t know what it means

https://en.wikipedia.org/wiki/Cockle_(bivalve)

I think most people who aren’t familiar with them from real life know them from the nursery rhyme “Mary, Mary, Quite Contrary.” If you mean that you don’t understand it as a lyrical allusion, then neither do I.

The reason I even cared enough to listen to a bunch of different versions and post about it was that I heard Gilmour singing “cockleshell” and just took for granted that it was wrong. I was initially just going to post it as a weird flubbed lyric but then I Googled it to see if other fans talked about it online, and ended up going down a rabbit hole.