Laying on your back🤔

Ashlee • R.I.P my little angel 4-8-2016 💕 Welcome to the world Aria, our Rainbow baby 🌈🍼 Esthetician💄
I am 22 weeks pregnant.
Im not really 100% clear on the whole "dont lay on your back while pregnant" thing, can anyone fill me in please? Because i still do it 😞 and i need to know the harm.
What is the harm?
What happens if i do?
What if its not to sleep, just to relax or get comfortable?
How does it effect?