That doesn't have to do with any idom, people can agree on anything they want to.
Things that people can agree on means things we all think are true, such as scientific facts. You also hear this used when the speaker is hoping that people will agree with him, as in "I think we can all agree on this!"
This is not an idiom. It means exactly what it says. Agreeing means that people perceive things in the same way and have the same opinion on it, or to consent to do what another person says. Things people can agree on are things that they can all consent to do, or that they all have the same opinion on.
An idiom for things people can agree on
To "call it even" means to agree that there is no winner in a contest, or no debtor in an exchange.
forget the things that have pass
to have more important things to do
This is not used as much as it once was, but people will know what you mean if you use it; it's not obsolete.
It's not an idiom. To break camp means to break it up, to pack your things and leave the area. It can be used as slang, however, to mean a group "packing up" and leaving.
It means that people are unable to agree on something. They are at an impass.
This is not an idiom. When you see AS ___ AS ___ you are looking at A Simile. This is comparing two flat things.
You gain "brownie points" by doing nice things for people who will be able to help you in the future.
It means that the person is slow to catch on... isn't understanding things as quickly as other people.
To "call it even" means to agree that there is no winner in a contest, or no debtor in an exchange.
It means things are trustworthy.
Fair play is not an idiom - it means exactly what it says. Things are going along fairly.
forget the things that have pass
to have more important things to do
It means someone who is very good at things like gardening and growing things.
This is not used as much as it once was, but people will know what you mean if you use it; it's not obsolete.
It's not an idiom because you can figure it out. You're ahead of the rest of the people. You're doing things that others can't imagine doing, so you must have been born before your normal time to live.