The ProBro is that dude who knows your class better than you, tells you what to do, and gets angry when he dies in a fire because "stupid healer" and "stupid tank," as he might yell in party chat.
The ProBro is usually good at his role, so long as he is fighting a carcass that won't defend itself. ProBros tend to group up in clusters of 100s of people in what is called a Free Company. This isn't to say that Free Companies are bad; quite the contrary. You can usually tell if you're dealing with a ProBro because his Free Company's name is flashy and indicative of true power zerging skill. Examples of such names might be "Crits and Giggles," where if you don't have the crits, there'll be no giggles.
ProBros come in all shapes and sizes, and will stare at you intently until they get the buff or reaction they are looking for (BTW, can we please remove the stare animation? It is really creepy, especially from that impish race). They aren't just limited to DPS; nay--there are plenty of ProBro healers and tanks too! These little guys are most known for their inability to stay in a group if even the slightest thing goes wrong. For them, it is no problem to ride their chocobo around the world while waiting for another group to gather. It is estimated that there is exactly 1 ProBro tank for every 1 ProBro healer for every 2,435,643.2 ProBro DPSers.
You should approach ProBros with caution. Stroke their ego with warm milk and cookies. Remind them how awesome they are and how lucky you feel to be in their presence. When they die (often), encourage them to build up resistance and stamina with physical activity, such as running back to the group. They'll be mega super buff in no time, and those cleaves they were eating will quickly become nothing more than flesh wounds.
Never ever ever feed a ProBro. They have a highly specialized diet, and to feed them anything contrary is to expose them and you to painful flatulence. If you do decide to toss them a snack, make sure it includes a nice tall glass of Positivitea.
Remember, ProBros are important members of our community. They bring years of experience--not only in game, but life--that we can learn from. They will help you burn even the mightiest of dragons with the fury of their hotkeys. All they require is a little TLC, and they will make you proud of being who they want you to be.