Lists are a series of values stored together as a data structure. On their own they’re not any more useful than regular Sass variables, however they become more useful when using the built-in functions Sass provides for working with them.
If you want to set a font-stack as a variable in Sass you could do something like $fonts: Helvetica, Arial, sans-serif in a single variable. Later you could assign the font stack with font-family: $fonts; However what if you only wanted to assign one of the fonts and not the whole stack? Sass lists might be your answer.