Abuelita is a Spanish word for grandma; it is actually a diminutive form of abuela, and so would most literally translate as "little grandmother", but in practice it is used to express affection, rather than diminutive size. Because of this, it is common to translate it as "granny", although I have to admit, as a native English speaker, I did not realize that granny was more affectionate in tone than the more common grandma.
Meanwhile, Abuelita is also the brand name for Nestle's 'Authentic Mexican Hot Chocolate Tablets'. These are hard chunks of sweet chocolate flavored with cinnamon and vanilla which are melted into into milk.
These are not uncommon in America, but are hardly popular in the average American household. The major strike against them is that they need to be melted on the stovetop, with a firm whisking to make sure the chocolate is fully melted and blended in. Further anti-American behaviors include forming a light skin on top of the drink as it cools, and mixing cinnamon into perfectly good chocolate.
However, if you can bring yourself to overlook these flaws, Mexican chocolate is worth the time it takes to make it, and most people start to like the cinnamon by the second cup. And if you are curious to give it a try, Abuelita chocolate has the unique advantage of being sold everywhere. Well, almost everywhere. Most large grocery stores will have it, although you may have to look in the ethnic food section. Wal-mart carries it. And just about any local tienda will carry it.
In America Abuelita's main competitor is Ibarra brand table chocolate, which in my experience is slightly more expensive and tastes approximately the same (but YMMV). Regardless of which brand you buy, they are both vegan and both available in powdered form, which I have not tried and so can neither recommend nor condemn.