What does this horizontal bar at the first measure mean?

That is a multi-measure rest. The numeral 15 above it indicates that it is equivalent to writing out fifteen bars of full-measure rests. This is done to save space, for example in orchestral parts. https://music.stackexchange.com/questions/81899/what-does-this-horizontal-bar-at-the-first-measure-mean/81900#81900.

1. What does a bar means in music?
2. What is a horizontal bar called?
3. What does a horizontal line under a note mean?

What does a bar means in music?

Bars/ Time signatures

Defined by Mighty Expert: 'a bar is one small segment of music that holds a number of beats. Multiple beats make up a bar and multiple bars make up a song. ' The number of beats in a bar is wholly dependent on the time signature of the song you are writing.

What is a horizontal bar called?

The horizontal bar, also known as the high bar, is an apparatus used by male gymnasts in artistic gymnastics. It traditionally consists of a cylindrical metal (typically steel) bar that is rigidly held above and parallel to the floor by a system of cables and stiff vertical supports.

What does a horizontal line under a note mean?

In musical notation, tenuto (Italian, past participle of tenere, "to hold"), denoted as a horizontal bar adjacent a note, is a direction for the performer to hold or sustain a note for its full length. ... Either way, the tenuto marking indicates that a note should receive some degree of emphasis.