Definition of suited
Suited means "matched". When we talk about suited cards, we are talking about cards that are the same suit. For example, 56 of spades or 89 of hearts. These hands can be written as 56s. A distinction must be made between 56 without any other mention, which means that it is 56 mismatched (e.g. clubs hearts or spades diamonds) and 56s. Suited cards will make a flush at a higher frequency than unmatched cards. In general, suited cards have 5% more equity than their non suited counterparts. For example, 78s will have an overall equity (against a predefined range) of 5% more than if the hand were offsuited (78, not suited).
Suited cards, perfect for semi-bluffing
The advantage of suited cards is that they will make a flush at a higher rate than non-suited cards, but they are also perfect for making a semi-bluff. For example, if you have a hand like 78 in spades and the flop comes 5-9-2 with two spades, you will have a great hand to raise on the flop in a semi-bluff with your gutshot and flush draw. These cards can be played more aggressively.