Pop-Tarts are a breakfast thing, yes. They are just the right size to fit in a toaster so you can have a warm something in the morning, without having to cook. They can be filled with apple or sprinkled with cinnamon. There are many flavors, actually. You can’t make one of these things, so you have to go to the store to get more. Some people have a very strong liking to them.
It does not really mean to go without breakfast, though. It means to go without a treat that most people would not think of as a very good breakfast. Some would call it a “guilty pleasure”, something you want, but probably should not have very often.
A Pop Tart is a thin breakfast pastry, about the size of a piece of bread,with a very thin fruit filling manufactured by the Kellogg Company. They fit right into a toaster. Kellogg has been making great cereals like Corn Flakes, Rice Krispies, etc. for a very long time. Pop Tarts were introduced in the late '60,s I think.
They used to have in Britain certainly in the late 80s, early 90s. They were lethal little things, the filling would get so hot in the toaster that one had to be very, very careful when biting into them. There taste was not something I’d recommend, but my son did eat them occasionally.