Simplifying geometry first of all, let me tell you I'm a total novice in 3d modelling, my experience is with Sketchup only. 
Recently, I came in need of a particular model, and I got it in a STL format. 
Sketchup imported it just fine, however, the mesh is so loaded with unnecessary details, it makes the application just freeze.
I was told Blender can solve this problem, so I came here to ask you how can I make it a clean mesh, without losing detail but making it lighter so the software can run (and no, it's not a hardware problem, I'm using a GTX 1070 with 16gb ram and up to date processor)
This is what I'm talking about:
https://imgur.com/a/RC3U9Er
Sketchup can't handle that many faces.
So, how do I fix it using Blender?
There are a couple of things to reduce the meshes.

You can find the most part in the Mesh > Cleanup menu.

Remember you have to enter the Edit mode.

Also a Mesh > Faces > Tris to Quads helps to reduce the edges.

See the image below: