It has been introduced in the topic "Continous".
We mentioned when introducing the possessive aspect of 있다 that there are other ways to indicate the possessive in Korean. Here we teach the verbs 가지다 (sometimes 갖다) and 들다, with 가지다 the more common of the two. Both these verbs can be translated as "to hold" or "to carry." 들다 additionally can mean "to pick up" or "to take in hand."
In the continuous aspect, both these verbs become "to have." This is more specific, and restrictive, than using 있다.
While "A는 B가 있다" in general indicated that A possesses B, "A가 B를 가지고 있다" implies that A is carrying around B. This makes sense if it's something in your bag or your pocket, but it makes less sense when you're talking about your house or your friend.