If you look at https://docs.blender.org/api/current/bpy.types.Key.html#bpy.types.Key you see that shapekeys in the code you cite is a bpy_prop_collection of ShapeKey.
So the code “for j in shapekeys:” will be a loop where j gets assigned each element ShapeKey of the collection in turn. That ShapeKey element has a “name” field (a string), so it can be assigned, as it is here.
The rest of the code is hard to understand without context. What is “names”? It looks like it should be a string, since it assigned to j.name. But the code “if shape.name in names” is hard to figure out. What is “shape” in this code, and consequently what is “shape.name”?
The expression “x in y” in Python can mean a number of things, depending on what y is. If y is a string, as it seems to be here, then it treats the string as a vector of characters, and asks “is the character shape.name one of the characters in names?”. I rather doubt that shape.name is a character, so I am just left confused by this code.