When you walk into a bathroom after someone has just taken a terrible shit and it smells completely disgusting. You now "Own it". That smell belongs to you even though you didn't produce it. If someone sees you leaving the bathroom they are going to assume you did it. It's not even worth explaining, just except the fact that it now belongs to you.
I went to the bathroom at work this morning after someone had just taken a horrible shit. In leaving I passed a co-worker who naturally thought I produced that smell. But now he is going to "Own it".
The same rule applies when plunging out someone else's "Log Jam" - some asshole who plugged up the toilet just before you walked in. You now own that one too!