This is the 4700th most frequent French word.
pleut
"Pleut" means "rains" (third-person singular of "to rain").
In this sentence, 'pleut' is used as a verb in the present tense indicating the current weather, specifically rain.
Il pleut beaucoup aujourd'hui.
It is raining a lot today.
Here, 'pleut' provides the reasoning for the action described, again as a verb describing the weather condition.
Je reste à la maison car il pleut.
I am staying home because it is raining.
The word 'pleuve' is the subjunctive form of 'pleut', used here within a subordinate clause expressing a condition.