It means "interval" roughly you can say “Distance”.
“If you hit or getting hit” it’s all about maai. It’s very very important I can say. One must have to maintain a good pocket of space during combat.
Let me elaborate….
間 Maa means Space,Interval
合いAi means Suit, fit..
“Suitable space” is the suitable term for you to perfectly understand and we can say “proper distance” too.
Maai is a proper physical distance between two antagonists. It might be said that keeping good maai is creating enough space to manoeuvre.
However, in terms of distance, if either the defender or attacker is able to strike the other with an attack without moving the feet, then maai is too short. Good maai is when the attacker can strike the defender by moving a half step forward or the defender can retreat out of range with a half step backward. Of course, this maai constantly fluctuates during fighting and both attacker and defender must be aware of the potential range of his/her own and the opponent's techniques and adjust the distance accordingly.
Peace and harmony,