It's a catchphrase that's become popular lately. The idea is that you are in a bar, and someone who is drinking is about to try something dangerous--possibly start a fight with someone. The person who says "hold my beer" is suggesting that he needs both hands free. But he is also suggesting it is something easy that isn't going to take long. He'll be back in a minute to finish his beer. So the situation is that someone is about to do something reckless, difficult, and dangerous, but hasn't prepared for it and is treating it as something fast and easy. He might be doing this because he has bad judgement due to drinking, or because is he is bragging, or both.

"Hold My Beer is an catchphrase said before attempting a dangerous and/or ill-advised stunt. Online, it is frequently used to categorize “Fail” videos, as well as to joke about concurrent and similar negative news stories" Thanks internet: knowyourmeme.com/memes/hold-my-beer