If your system is actually medium, you may get away with running most games available. Certainly, unreal projects are hogs and those are where the trouble will reside. Most games today, specifically unity games, feature graphic settings to accommodate lower end pcs. This is a crux of the argument though, while your pc won't struggle as hard, the game will probably look like shit because of it. Personally I consider my own system to be medium at best, and it's capable of everything but those fucking dynamic shadows. Shadows always tank my frames.
If you are using a low spec pc, to simply put it, you should upgrade. It's 2018, most low budget pcs today are still pretty damn good in comparison to early 2000 pcs. Considering you did not share your own specs, I won't make any assumptions.
I have seen some videos on youtube about people running modern games to render everything as low as possible.
They achieved their result by using Nvidia Profile Inspector.
That's probably what you're looking for in relation to your question.