A mono sound has no difference between the output of the left and right channel, so it sits in the "middle" of your mix is, so to speak.
A stereo sound has slight differences between the left and right channel, so you end up with something that sounds "bigger" so to speak - it doesn't sit directly in the middle, instead its spread out over the left and right channels.
Obviously this sounds like witchcraft because if you really think about it, a sound that comes out of both your left and right channels equally should sound just as big as any other sound that comes out of your left and right channels equally. Stereo is more or less a psychoacoustic effect though - you're tricking your brain. Don't worry about how it works too much ahaha.
Sub bass should always be in mono (to avoid phase cancellation issues), but it can be helpful to have supporting elements of your song like chords and pads and stuff in stereo, because they kind of fill in the space, so to speak, and then they don't sit in the middle and take attention away from your melody or midrange synths.
You can simply leave the sound in a stereo channel, but if you want to make a sound "bigger" than it already is, you have to use an effect of some kind (or do some panning) - these include stereo delays, flangers, chorus, phasers, ensemble effects, and reverbs.
Usually if I want to make a sound bigger I just add a reverb unit to it. This is the easiest way to do it.
Another common trick is the Haas effect - you add a stereo delay to a track, and set the delays to say, 30 ms on the right channel and 40 ms on the left. This creates a tiny difference between the left and right channels, but the result is that it tricks you into thinking the sound is bigger than it already is.
