When you look at the notes on a particular scale, each note can be labeled with a scale degree. The bottom note on the scale, called the tonic, is the most important note on that scale and can also be called the tonal centre. The tonal centre, combined with the 5th note above it and the 4th note below it form the tonal chord, which is the most important chord in that particular piece of music.
The tonic is the lowest note of the scale, and the other notes of the scale are defined by their relationship to the tonic. As you go up the scale from the tonic, the notes are called the supertonic, mediant, subdominant, dominant, submediant and leading tones.
When composing music, composers tend to move away from the tonal centre and then back again to make the piece more interesting, and to create movement by alternating the use of chords that are dissonant with the tonal centre chords that are consonant with the tonal centre. The tonal chord is also used to close sections of the musical piece that are important, as well as to end the piece of music.
Basically, the tonal chord is the chord which much of the piece of music is composed around. The relationships of the other notes and chords to the tonal note are what add interest to the musical composition. There are a wide variety of different types and relationships that are used by composers who are writing tonal music.
During the course of a piece of music, the composer can change the tonal center, and there can also be secondary tonal centres. There can even be more than one tonal centre during certain portions of a piece of music. However, a single chord based on the tonic should always appear to be the most final and stable in the piece of music in the minds of the people who listen to the music if you want the piece of music to be a successful composition. This is why it is so important to understand the idea of the tonic and the tonal chord.