so-so; passable; neither particularly good nor bad. Can be used as an adjectival noun or an adverb (meaning 'fairly' or 'not bad').
試験の結果はまずまずだった。
The exam results were so-so.
体調はまずまず回復してきた。
My health has been recovering fairly well.
ケーキの味はまずまずだが、見た目をもう少し工夫した方がいい。
The cake tastes okay, but it would be better to work on the appearance a bit more.