Metaphor of baseball and dating
first: the first step in a sexual relationship kissing with an open mouth or using tongue (french kissing) first and a half base: under the shirt, bra still on second: one step up from first base, heavy petting and feeling up while making out,direct oral or manual stimulation of a partner's breast/nipple, up the shirt or shirtless for both partners.
shortstop: in baseball, the shortstop is the field position between second and third base.
The fact is that every person has heard a high schooler brag about his romantic conquest with the lines like “yeah, I totally went to second base with Michelle last you, you know it, ha-ha”: high fives his friend.
More specifically French kissing, as pecks aren’t that noteworthy.
So third base used to mean touching of the private parts below the waist, and now a lot of people regard it as oral sex.
And with this terminology, logically, home-run means full sexual intercourse.
But since you Googled “what are the bases in dating” you must either be a non-US citizen or eight years old, in which case you should be quick as your older brother might get into the room and you’ll have to endure years of getting made fun of brutally for this search. But in case you’re not from the North American continent or Japan and have the luck of not knowing anything about baseball, this terminology can be too confusing and you end up knowing only that something sexual happened between two people. Baseball is a deeply complicated game with a lot of “ifs” and “but ifs” so it cannot be explained in great detail here, nor is that why you are here for.
But you need to know some rules of this sport so you can understand the metaphor.