Not enough disk space, even tho I've got nearly no files at Mac

I get notifications about my disk space permanently, saying that I ain't got enough space. At first, I deleted most of my files /I kept something about 2 Gb and a few applications/, but still got just something about 5 Gb free disc space left = which leads me to the beginning, that I ain't got no space. I'm sure it's because I have my Macintosh disc "separated" /as you can see on the pictures below/, but how come that this hasn't been a problem a couple months, maybe a year ago? /since i've got the Yosemite OS X/. Just to be clear, I get those notifications maybe for a month. So, I have 2 questions.

1. When I check my disc space, it says that "Other" is nearly 85 Gb big. What does "Other" mean, and how can I get rid of it /clear, delete it/?

2. I'd like to update my OS X into Sierra, but when I try to do so, it says that I ain't got the space to do it /I'm sure it's because there's no free disc space on the "first" Macintosh HD -check the pic. below/. How to fix this one?

I hope you understood what I was trying to say here, my english isn't so good, so... Thank you anyway