An idiom meaning the best one can do or the maximum one can expect; no more than that.
初めての試験で合格が取れれば関の山だと思っている。
I think getting a pass on my first exam would be the best I can hope for.
彼の実力では昇進は無理で、残業代で生活できるのが関の山だ。
Given his ability, promotion is unlikely; at best he can live off overtime pay.
この安い材料で作る料理はおいしくなるのが関の山だ。
With these cheap ingredients, making something delicious is the best you can expect.