In it's simplest form, passive losses are those from businesses/activities you invest I , but in which you don't participate. It's somewhat similar conceptually to capital gains…
Ex…I gave my neighbor $10. He used it to open a lemonade stand. I don't help him at the stand or advise him in any way. This is a passive activity. If his lemonade poisons people, I probably won't be found liable.
Vs.
I gave my neighbor $10. I advised him on where to set up, I picked the mix and I worked several shifts and we split the profits. If our lemonade poisons people, I probably will be sued as a partner.
Relate it back to partnerships…a silent partner (not active) has passive losses. He can't be sued.
In terms of rental it's the same thing…
My dad has 4 apartments he bought as investments. He has a property management office handle renting the properties – he's a passive investor.
He has 4 properties and he does the maintenance, collects rent, etc. Hes active.