Superscript numbers or variables usually denotes an exponent. They are written on the upper right corner of a number/variable. For instance:
Subscript numbers or letters usually denote another variable. Like, in physics:
denotes the final speed of a moving object.
denotes the initial speed of a moving object.
Subscript numbers may indicate the rank of a term in a sequence. For the sequence:
refers to the first term of the sequence (1)
refers to the sixth term of the sequence (8)
EDIT: I just want to add that there can be more meanings to subscripts. It depends in what situation does subscript numbers appear.