What does mean in a sheet music?

The symbol "—" in sheet music indicates a tie, which connects two or more notes of the same pitch. When notes are tied, their durations are added together, meaning that the resulting note will be held for the total duration of the tied notes.

For example, if a quarter note is tied to an eighth note, the resulting note will be held for a duration of three eighth notes.

Ties are used to create legato playing, where notes are played smoothly and connected, without any noticeable gaps in between.