I typically write thrillers, so for me, the main plot is usually about events external to the character, whereas my subplots are about the internal side of things. That's not to say that the subplots are sitting around introspecting; it's that they usually arise from the character's personal interactions, rather than being the big machinery of the main plot and the antagonist.
Something that's important to me is that the subplot shouldn't be a tangent. It should intertwine with the main plot and provide complications as well as contrast. Say, for example, that what the protagonist wants to do to resolve the subplot will damage their progress with the main plot, or vice versa - that creates some interesting tension.
To put some bones on that, let's take a (fairly standard) story where you have a career criminal who wants to reconnect with his family, but is forced back into a Life Of Crime (tm). So your external plot events, the A plot, are focused around the criminal element, and the internal plot events, the B plot, are the family*. Because these two elements are inherently opposed, you can use them to create conflict. The classic example for this type of story would be a family gathering like a birthday party; our protagonist turns up intending to focus on the B plot, but then one of his criminal buddies (from the A plot) turns up at the party. Instant conflict and the chance for a dramatic scene.
The classic way to escalate this type of story is to have the two (still opposed) plots combine at the end of the story. The antagonist takes the family hostage, that sort of thing**. With that, resolving both the A plot and B plot will require the same action from the protagonist. For extra bonus points, the protagonist could defeat the antagonist with something they've learned from the B plot, e.g. in reconnecting with their kids, the MC has learned that their daughter can sing a high note that shatters glass, so the MC get the daughter to do that and uses the distraction to save the day.
So far, so by the numbers. Let's take another example of a B story: the love interest. In THE MATRIX, the A story is about Neo coming to believe he is The One. The B story is him falling in love with Trinity. While these two elements are not in conflict like the example above, they are again on the lines of external plot action vs internal plot action. Here, Neo - a loner at the opening of the film - can only triumph thanks to Trinity's love, which literally brings him back from the dead when the A and B stories cross at the end. It is only thanks to the B story that the protagonist can complete the A story. The B story is not as central to THE MATRIX as in the example above, but it does pop up at all the most vital moments; Trinity's interactions with Neo almost always push him forward into the main action.
I'd note that these are somewhat formulaic examples, and there are probably folks reading this who are thinking "but my work is more complicated than that", but it helps to talk in clichés when discussing plot devices since clichés provide very clear examples. A good part of the fun of writing (for me, at least) is working with far less clear examples, inversions, etc., but those require a lot more analysis and are tough to work with when you're not familiar with the kind of techniques we're discussing. As with all "the rules", once you have an understanding of the principles behind the simple examples, you can start bending and applying them with care to more complex work.
*: you could make a less cliched story by inverting this priority; having the family and relationship be the A plot and driving the majority of the action, with the recidivism element being a B story.
**: ANT-MAN (2015) follows this template exactly, but throws in a "C" story as well: the original inventor of the shrinking suit, Hank Pym, and his relationship with his wife, Janet. The protagonist, Scott, saves the day by learning from Janet's example, rather than necessarily something from his B plot. I'm not sure this is as successful, though $500 million at the box office argues otherwise. (Honestly, the antagonist is more related to Hank than Scott in the first place, which is one of the other reasons I don't think the story quite hangs together - but again, that's just me.)