So, in a situation where you 're going out to dinner or you 're going out to any kind of meal,

like lunch or breakfast or whatever, with some other people

and you actually want to treat those people.

So you can say, " I 'll pay," like if they 're saying, " Oh, I don't really have much money,"

or " How would you be able to afford this really nice restaurant? "

Like, " I can't afford this."

Then you would say, " Oh, it's okay, I 'll pay, I 'll pay."

But it's much more native, natural and conversational to say, " It's on me ". It's on me."

So, " it's " being the meal, so " the meal " is on me.

The meal is on me.