Confusingly the name plantain is shared by two very different generae of plants - plantago and musa.
The genus plantago consists of a group of rather unimpressive looking herbaceous plants which grow almost literally anywhere on the planet that has enough moisture. Despite their appearance, they are generally quite useful in herbalism having a variety of properties useful in treating wounds and insect bites1, some food value and significant abilities to ease bowel problems when taken internally2.
Genus Musa, on the other hand, are the family of tropical plants, related to and including the banana - although the name plantain is generally reserved for the non-sweet, red and green varieties. These generate characteristic, elongated fruit rich in potassium and either fruit sugars (in the case of the yellow banana) or starch (for other plantains). Being capable of providing good quality starch all year around, the cooking plantain is typically a staple crop for those cultures that use it.
How two such dissimilar families of plant came to share a name is far from clear.