The highest-rated Urban Dictionary definition describes second base as “when a guy feels up a female's breast over clothing, under clothing, or under the bra” (Heteronormative, much?). Next up: “Heavy petting and feeling up while making out, up the shirt or shirtless for both partners." Then, the confusion comes in: the most popular definition for “heavy petting” is “foreplay ranging from breast massage to masturbating one's partner.” That's a pretty wide range.
Another Urban Dictionary definition for second base differentiates between “major league” second base and “minor league” second base: minor league is “feeling a girl up,” while major league is “fingering or a hand job.” Again, pretty big difference!
Maybe it’s because of the confusion or the heteronormativity, but this sexual metaphor seems to be on its way out. Last year, a popular meme defined first base as sex, and then each succeeding base showed, as Know Your Meme puts it, “increasing levels of emotional intimacy.” For example, as @knifexparty put it, “First base: sex. Second base: actually hanging out again. Third base: seeing me cry. Fourth base: unconditional love & support.”
Luckily, there’s no real reason you need to use baseball metaphors for sex when there are so many other words available. If your friend tells you, “I got to second base!” you can always answer with, “Ooh, you had a second date??” — or you can just ask what, exactly, they mean.