Pleuvoir is the infinitive. You use the infinitive because you have already conjugated the verb aller in the sentence "Il va pleuvoir." When there are two verbs on after the other, the first is conjugated and the second is in the infinitive The va + infinitive is used when something will happen in the near future. For example, you look outside and notice the sky is grey and say "Il va pleuvoir."

"Il va pleuvoir" is in the near future (It is going to rain) You conjugate the near future using: - the verb 'aller' in the present tense (vais, vas, va, allons, allez, vont) + a verb in the infinitive Here, the verb in the infinitive is 'pleuvoir' (to rain). "Il pleuvra" is in the future simple (It will rain) "Il pleuvait" in in the imperfect (It was raining) And if you ever need them, here are a couple more: "Il pleut" is in the present tense (it's raining) "Il a plu" is in the passé composé, past tense (it rained) Hope it makes more sense now :)

"il va pleuvoir" (it is going to rain) and "il pleuvra" (it will rain) are two types of future tense.