A harmonic of a note is a higher note that is related to it by a simple multiplier. ie. the second harmonic of note X has a frequency twice as high; the third harmonic is three times as high, etc.

The second harmonic of a note on the guitar will be at the mid-point of the string. So, the second harmonic of an open string is at the 12th fret. The second harmonic of a string fretted at the sixth fret is at the fifteenth fret.

The more accurate you are about finding the right point on the string, the louder the harmonic will be.

Another big factor is your touch. You need to touch the string just enough to set up the harmonic vibraton on either side of your fingertip, without touching it so hard that you damp it out.