Pax et bonum!
We have a sentence: De gustibus non est disputandum.
I understand the meaning of it but I just don't really understand disputandum.
Please explain this to me.
You are correct in identifying "disputandum" as a gerundive. Actually, in conjunction with a form of the verb "esse" (here "est"), the gerundive forms what is known as the Second (or Passive) Periphrastic construction, denoting obligation or necessity. For example, "Caesar interficiendus est" (Caesar must be killed).
Here "disputandum" is being used in an impersonal construction, which explains the use of the neuter in the gerundive. One could render into English: "there is not to be a disputing about tastes." Alternatively, the sentence could have been cast in a personal construction by making "gustus" the subject of the gerundive/periphrastic construction: "Gustus non sunt disputandi" (tastes are not to be disputed). English does not have the flexibility of Latin in this regard, so the renderings in English are pretty awkward.
For further discussion of the Periphrastic Conjugations, see Allen & Greenough's New Latin Grammar (available on line), sections 193-196. For further discussion of the Impersonal Construction with intransitive verbs, see section 208d.