prefetch_related() 에 설명이 잘 나와있습니다.

This has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.

select_related works by creating an SQL join and including the fields of the related object in the SELECT statement. For this reason, select_related gets the related objects in the same database query. However, to avoid the much larger result set that would result from joining across a ‘many’ relationship, select_related is limited to single-valued relationships - foreign key and one-to-one.

prefetch_related, on the other hand, does a separate lookup for each relationship, and does the ‘joining’ in Python. This allows it to prefetch many-to-many and many-to-one objects, which cannot be done using select_related, in addition to the foreign key and one-to-one relationships that are supported by select_related. It also supports prefetching of GenericRelation and GenericForeignKey, however, it must be restricted to a homogeneous set of results. For example, prefetching objects referenced by a GenericForeignKey is only supported if the query is restricted to one ContentType.

한글로 대략 요약하면

1. select_related는 SQL join문을 만들어, related object의 field를 SELECT 문에 포함하는 방식을 쓰고,
2. prefetch_related는 각 relationship을 갖고와 python에서 joining 하는 방식을 씁니다.

아무래도 SQL문을 바로 실행하는 쪽(1번)이 언어에서 join을 하는 것보다(2번) 빠릅니다. 대신 select_related는 foreign-key/1:1 relation ship 에서만 쓸 수 있습니다.