Ell is a fancy name for a shape that looks like a letter L. And architecture means designing and building things like houses and buildings.
So when people talk about ell in architecture, they usually mean a building or room that has an L shape. It could be a house where one part of it goes to the left and the other part goes to the right. Sometimes, people like to build things in an L shape to make the space more interesting or to fit the design of the land that the building is on.
So if you ever hear someone talking about "ell" in architecture, just think of the letter L shape!