In Reaper, LPF/HPF is really easy. The basic jist of them is:
Low Pass: Everything below a certain frequency "passes", or gets through.
High Pass: Everything above a certain frequency, same thing.
In Reaper, just add ReaEQ to the FX chain of the track/bus, and select Low Pass/High Pass for the type. Ignore the middle bits, it's just freq #1 and #4 that I'm talking about
Here's a high pass at 100hz. This would pull some of the lower low end out of your rhythm guitars, and give the bass more room to breathe down there so that they don't step on each other:
And a low pass at 12000Hz. It pulls some shimmer/top end from the upper end of the guitar tone where it tends to sound brittle.
Sometimes I'll low pass a lot lower, like 6-8k because I like dark guitars. And if I'm using a 7 string, I might move the high pass down to 80hz, to let the low end come through a bit more. There is no real perfect setting, it all depends on your tone and the mix you're going for.
Simply put (oversimplified really) if you "cut off" everything below 100hz from your rhythm tracks, and your bass track is nice and full at 100hz, then the bass can breathe in the mix without the mud from the rhythms stepping on it in that general space.
Check post #5 here too for some basic tips: