Meantime and Meanwhile basically have the same meaning—“(at) the same time,” “during a certain period of time.” But their grammar is different.
Both words can be used as nouns, but only meanwhile can be used as an adverbial—a word that modifies a whole sentence or its main verb:
You go stand in line to get a table for dinner; meanwhile, we will park the car.
Meantime must be used as a noun, but with the same function:
In the meantime, we will park the car.
You might occasionally hear or see the phrase for the meantime, which is essentially the same phrase as in the meantime—but far less common.
Meanwhile usually appears only at the beginning of sentences, while in the meantime often also appears at the end:
We will park the car in the meantime.