https://en.wikipedia.org/wiki/Baseball_metaphors_for_sex

"Among the most commonly used metaphors are the bases describing levels of physical intimacy (generally from a heterosexual perspective). Definitions vary, but the following are typical usages of the terms:

First base – mouth-to-mouth kissing, especially French kissing;
Second base – skin-to-skin touching/kissing of the breasts; in some contexts, it may instead refer to touching any erogenous zones through the clothes (i.e., not actually touching the skin), or manual stimulation of the genitals;
Third base – touching below the waist (without sexual intercourse); in some contexts, it may instead refer to oral stimulation of the genitals;
Home run (home base or scoring) – "full" sexual intercourse.