MATT MCGINN does as good a job as any.
A "Piece" in Scotland, is a sandwich.
A piece an' jam is a jam sandwich a piece an' cheese etc..
It also becomes a generic term for what North Americans call a "Packed Lunch".
Thus: "Whit's oan yer piece?" = "What did you bring for lunch today?"
Jeely is of course "Jelly" ie. the clear seedless jam.
Not to be confused with "Jelly" which is served as dessert and referred to as "Jello" on some continents.
"I like aeroplane jelly, aeroplane jelly for me" Dating back to a time when aircraft were kept in flight by something that we could actually see because the windows matched the seat rows and you could tell if one wasn't working.