What is indicated by a 'tie' in musical notation?

A 'tie' in musical notation is used to link two notes of the same pitch, combining their durations into one sustained sound. This means that instead of playing the two notes separately, the performer sustains the first note for the length of both notes. This notation is essential in creating phrases or extending the duration of a single pitch, allowing for a smoother, more connected musical line.

The other choices do not apply because a tie specifically indicates a connection of duration rather than instructing players to repeat notes, play them distinctly, or change the tempo. Each of those concepts has different notational symbols and implications within music.