The video explains SQL joins using Venn diagrams and two tables: customer and city. It focuses on inner join, which returns only rows with matching values in both tables based on city ID. For example, customers with city IDs in the city table, like Bob, Sally, and Tom Smith, are included, while those without, like Mary Smith, are excluded. The presenter illustrates writing an inner join query to fetch these matching records, emphasizing how joins interact based on common data.