In a nutshell
The entity-relationship model is a picture of the data before any tables exist. You draw the real-world things you care about (entities) as rectangles, the facts about them (attributes) as ovals, and the links between them (relationships) as diamonds, labelling each link with how many connect to how many (cardinality). Then a mechanical recipe turns that diagram into actual tables — which is why join tables appear for many-to-many links.