A colloquial phrase expressing mild resignation or dismissal, like 'oh well' or 'never mind'.
忘れ物に気づいたけど、まあいいや、今日は仕方ない。
I noticed I forgot something, but oh well — it can't be helped today.
財布を家に置いてきたけど、安いからまあいいやと諦めた。
I left my wallet at home, but since it's cheap I thought 'oh well' and gave up.
彼が遅刻しても、もう待つのは面倒で『まあいいや』と言った。
Even though he was late, I said 'oh well' because waiting was too much trouble.