[WAKARI-MASEN]

It means 'I don't get it, I don't understand, I have no idea'.

WAKARI-MASU = I get it.

WAKARI-MASHITA = I got it.

WAKA-RANAI = I don't get it. (casual)

WAKA-RU = I get it. (casual)

WAKATTA = I got it. (casual)