Often, when a guitarist doesn’t press hard enough on the strings or presses down too far from the frets, the guitar will produce a buzzing sound that comes from the string vibrating on one of the raised metal frets.
Other potential causes of fret buzz could be that your guitar needs:
a re-fret job: Frets can become uneven or worn down for various reasons. In this case, you’ll need a fret dressing to fix the buzz.
a nut / truss rod / action adjustment: There are a few reasons that the height of the strings in relation to the guitar neck may be off and causing fret buzz. If the strings are too close to the frets, you’ll have fret buzz regardless of playing technique. If the strings are too far from the neck, it will be much more difficult to press hard enough on the strings. If you aren’t sure what’s causing fret buzz and you're playing with proper technique, take your guitar to a trusted guitar tech for a diagnosis.
new strings: If your strings are too long overdue for a change. They may have built up a grimy film that contributes to fret buzz.