Your hands are the only thing connecting you and the golf club, therefore is one of the most important fundamentals.
Your grip has its main influence over the clubface which consequently controls the direction of your shots and can be either ‘Neutral’, ‘Strong’ or ‘Weak’.
A Neutral grip is a key to a more neutral clubface through impact. The lines formed by the thumbs and forefingers (represented by the yellow lines) should be parallel with the forearm connecting the bottom hand on the grip.
A strong grip can be when you can see 3-4 knuckles in the glove hand, and/or the bottom hand creating a line between the thumb and forefinger which points outside the shoulder of that arm. It is also common to see the thumb on the glove hand if the club is held with a strong grip. A trait of a strong grip is a draw/hook spin on the golf ball.
A weak grip can occur when you can only see the first knuckle of your glove hand, and/or the bottom hand creating a line towards the shoulder closest to the target. A trait of a weak grip is a fade/slice spin on the golf ball.