Pick a Table
Every table the build has loaded lives on the left. Nothing on this page is stored here: the rows arrive from your Mac when you ask for them.
What a line in the model means. Nothing in these files says that two tables are related, so a line is drawn where two tables share a column name that no other table carries. That is a shape worth looking at, not a fact. Click a line and it counts the real thing: how many distinct values each side holds, and how many rows find no match. Show Every Line widens it to every shared name, which wires almost everything to almost everything and is why it is not the default.