Core answer
Start here. This is the direct spoken answer to practice first.
Why this matters
N+1 queries turn one list request into one initial query plus repeated related-data queries. The endpoint may look correct in development yet become slow and database-heavy as row counts grow.
The N+1 problem happens when the application runs one query to get a list and then one extra query for each item in that list. For example, it loads 100 orders and then queries customer details separately for every order. In EF Core this often appears with lazy loading or navigation properties accessed inside a loop. The fix is to shape the data with a projection or load the needed relationships intentionally.