This letter: has a sound somewhere between the English k as in kiss and g as in guess.
Put your index and middle fingers on your throat and say just the k- sound of the word "kiss." Now say just the hard g- sound of the word "guess." Feel the vibration of your vocal cords with the g-? That vibration that you felt is called voicing. We say that k is an unvoiced consonant (no vibration) and g is a voiced consonant (buzzzz).
At the beginning of a word (the initial position) or at the end of a word (final position) the Korean letter sounds more like the k than the g; it's not voiced strongly if at all.
However, when you put it between two vowels (the medial position) its sound is much more strongly voiced like the g.
Note: this is a general rule; there are exceptions and other minor sub-rules which I'm not covering in this brief tutorial.