The shift on a typewriter is designed to move either the carriage or segment a preset distance, allowing the typewriter to print a different symbol with each key. It is usually used to print capital letters, but may also be used for symbols.
On 3-Bank Machines
Typewriters with only three rows of keys, or "three-bank" typewriters, will often have two shifts (often marked SHIFT and FIG. or FIGURES) to compensate for the reduced number of keys on the keyboard. This allows them to produce lowercase, capital, and symbols using the same key depending on which shift is being pressed.
The most common issues with the shifting mechanism have to do with improper alignment of the shifted types.