A chord is a combination of different notes played simultaneously. Their names represent chord formulas that define the notes to be played for every chord with each bass note. For example, the A maj3 (above) is comprised of the A, C#, and E notes as the maj3’s chord formula dictates.
Chord formulas can be expressed numerically using the number of semitones between the notes; maj3 = 4 3 5. The 4 3 5 formula is universal to all maj3 chords and defines the notes to be played for every maj3 chord. Maj3 is just the name man has given the 4 3 5 fromula so we don’t have to call it 4 3 5.