Usually, friends with benefits (also known as Fwb) means that those who already know each other casually engage in sexual/intimate activity without ever dating each other exclusively. Instead, they share a flirty sex life. To some, this is a perfectly acceptable thing. To others, it is not.
The question at hand is whether such relationships are healthy or not. There is no clear answer because it all depends on the individual and how he or she wants to see things. If you have a friend with benefits, there are a lot of advantages that come with it but then again, there are also a lot of disadvantages. For one, you will never know when you might run into your friend again. You can never tell if he or she will still be with you even if they are having a great time together.
For other people, though, Fwb is a good way of establishing friendships with benefits. Your friends may not want to take things further and become more intimate with you but they are your friends and will support you in whatever decision you make. Having an Fwb arrangement ensures that you can always have your friends around, and they are your support group when times are hard. In fact, this could also be the beginning of an unforgettable friendship because you would have started out as friends. So, if you are thinking of a casual fling but would want something more, consider getting a friends with benefits relationship instead.