Some functions can take a variety of types. For example, the sort function can take a list of any sortable type: a list of numbers, a list of text values, a list of pairs of dates and numbers, and so on. To denote this, we give the type a generic name, usually t, or a, b, c, which is shown in italics in this interface. You can read this as "any type, which we name t".
Sometimes a type may have constraints on it. These constraints are written above the function's type.