Ik begrijp het niet.
English: I do not understand it.
In Dutch, 'niet' and 'geen' are used to make sentences negative. They mean 'not' and 'no/none', but are used in different situations.
Use 'niet' to make verbs, adjectives, or adverbs negative. Use 'geen' to say there is no noun or to deny the existence or amount of something.
Ik begrijp het niet.
English: I do not understand it.
Wij hebben geen huisdier.
English: We do not have a pet.
Zij is niet boos.
English: She is not angry.
Hij drinkt geen koffie.
English: He does not drink coffee.
Het boek is niet interessant.
English: The book is not interesting.