Just to expand on what everyone has said, PrEP prevents the HIV virus from taking hold should it get in to your body. It's not surprising that you haven't heard a lot about it. It has only been approved for about two years.
There is a wealth of information on
PrEP at the CDC website.
At the same time it has left a lot of AIDS prevention organizations flummoxed because it's arrival means that condoms and abstinence are no longer the only two options for reducing the risk of HIV transmission to people who are currently HIV negative. If you have a lot of emotion invested in one of those methods (like the Catholic Church is tied to abstinence and the AIDS Healthcare Foundation is tied to condoms) then PrEP being available can feel like a threat.
The ultimate medical goal since AIDS was discovered has been finding a vaccine that will prevent the virus from spreading. PrEP is not a vaccine, but it is functionally a step in that direction. With a vaccine you take in once (or every few years) and the HIV virus can't live in your body and make you sick. With PrEP, you get that same preventative effect, but only as long as you take the medication. So, just like HIV+ people have to take their medication (the same pill, by the way) to keep the viral levels down, HIV negative men on PrEP have to take their pills every day as well - as long as they are sexually active with a possibly HIV+ partner.
The FDA recommends PrEP for anyone having sex with multiple partners without a condom, and for couples where one person is positive and the other isn't.
In their own ways, PrEP and condoms are equally effective - but they are such radically different approaches you have to take a lot of your own environmental and behavioral factors in to consideration.
What they have in common is that they only work as long as you use them according to according to directions. Some people are really inconsistent in taking pills and some are inconsistent with carrying and properly using condoms. Also PrEP is expensive in the US (though ususally covered by insurance), while condoms are relatively cheap. PrEP only protects you from HIV, while condoms protect your from multiple sexually transmitted diseases.
I will just finish with this personal story. My nephew is in his mid-20s, gay, and sexually active. I love him dearly, but he is not the most responsible person in the world. Sometimes. Last time we visited we were talking about politics and health care and he was telling me about how much he appreciated his own doctor who proscribes him PrEP.
Inside I just breathed a huge sigh of relief! I didn't have to worry about whether he was using condoms and being responsible - as long as he took those pills he was at least safe from the killer that took away what seemed to be half my generation of gay men. I was ready to throw him a party.
That's what PrEP can do. With one prescription, it can make parents and uncles sleep a lot sounder knowing that particular angel of death will pass by their children.