Mixto is a term used for Tequilas where other types of sugar have been added prior to fermentation. Mixtos are only required to be made from 51% Blue Agave sugars, and the other 49% can be made from any other kind of sugar - usually sugar cane or high fructose corn syrup.
Like anything, there are high and low ends of the quality spectrum. Unfortunately, at the low end in addition to other sugars, mixto producers sometimes add colourings, flavourings and thickeners. Because of this less than enviable medley of additions, Mixto’s are often considered a cheap, slightly nasty type of tequila that tend to be served via a shot glass before being chased with lime and salt to erase any flavour whatsoever. There are some good ones out there made with genuine quality however, but it’s a hit and miss affair to find them!
It’s worth noting that distilleries are not required to put “mixto” on the bottle. If a Tequila isn’t labelled “100% Agave”, then you can be sure it’s a mixto.