In western European tonal music of the 18th and 19th centuries, the tonic center was the most important of all the different tone centers which a composer used in a piece of music, with most pieces beginning and ending on the tonic, usually travelling to the dominant in between.
There can be major scales and minor scales. The tonic remains the same in these two different "modes," for a given key, wheareas scale degrees such as the third degree and the sixth degree are altered in the minor scale.
Howevever, this can be seen another way. Each Minor scale uses exactly the same set of notes (key signature) as some Major scale and vice-versa. The only difference is which of these notes is the tonic. For example, C major and A minor have no sharps or flats. Consequently, the tonic plays an important part in determining why music composed in a minor scale sounds different from music composed in a major scale.