Composers use a range of signs and symbols to convey how they want their music to be played. Learning to recognise these instructions is an important skill to develop.
Traditionally music would be passed down through a culture by listening to other musicians and learning and repeating it. There was no fixed way to write down what was played.
The way we write music, using a system of dots and lines, originated in the middle ages in the Catholic church.
Being able to write down musical notation allowed different people to sing melodies without having heard them before, which was particularly important in times before there was any way to record music.
The important parts of musical notation are:
time signature - this shows the number of beats per bar
key signature - this shows the key note and what sharps and flats to play
tempo - this shows how fast the music should be played
dynamics - this shows what volume the music should be played at
notation - this shows the pitch and duration of the notes