This is another way of saying that each note has two names. The interval between C and Eflat can be called either a minor third or an augmented second. It is considered more correct to call it a minor third because the general name is a third. There are two basic criteria to consider when learning how to read music. Pitch and rhythm.
Pitch is determined by the placement of the note in the staff.
Rhythm, or the duration of the note, is determined by the kind of note it is. I will deal with the five most common notes first. Every note has a corresponding rest with equal value. Half notes and half rests get two beats. Quarter notes and quarter rests get one beat.