As a new players, teams tend to walk up to each other in the middle of a lane, Civil War style, and let lose with their ultimates. The fight ends when one team is dead. Nobody will retreat, nobody will maneuver, and you will look back on it as the most absurd form of combat ever.
If your team is ahead and you have a team composition that supports initiation, then there is nothing wrong with frontal assault. However, if it isn’t going well, you need to use your judgement as to whether you need to retreat. There are very few situations that should result in either team being completely wiped out. Acceptable situations for your whole team dying are: defending your nexus and… defending your nexus. If it is neither of those situations, then nobody on your team had the good judgement to realize the fight was being lost and that they needed to retreat.
I realize that by saying this, I run the risk of teaching you to not fully engage and thereby never win a game. So let me add a nuance to my previous statement. If you engage — do so wholeheartedly. Really commit to the fight. Good players will leave abilities or summoner spells that they can use as an “out” if the fight turns sour. The main initiator probably has no hope of surviving a fight that doesn’t work out, but the ranged damage dealers have very few excuses.
As you get better, you’ll learn more advanced engagement tactics. The first is flanking.
I need to warn you that there are risks associated with flanking. You are dividing your team into two parts and if the enemy team has a timely engagement, they may overwhelm one of those parts faster than you can respond.
The goal of flanking is to avoid having to get past the enemy team’s tanks to reach their squishy back line damage dealers. The best flankers do high damage, have crowd control, and are very mobile.
Their whole team will attempt to peel for their carries immediately, so it is important for a coordinated engagement. One way to do this is to have three people in lane, standing out there in a very obvious way — preferably under a friendly tower. When they engage those people, having your flankers immediately engage the back line. Since their tanks have used their gap closers to jump on our main team, they will have a hard time retreating fast enough to protect their back line. The primary goal is to not flank until after they have spent their movement abilities. If that opportunity doesn’t come, you might lose your tower because you are split up. Or worse, they might realize you are flanking and have their whole team chase down your smaller force.
Trapping is where you come from behind an enemy team and trap them between your team and your towers. This requires finding a way to get behind them.
The risk with this maneuver is that if your team isn’t stronger, you might just get destroyed and then lose your towers because, in cutting off their retreat, you have also cut off your own retreat.
I think it is fine to do this if you are far ahead — say you were taking baron and they are trying to take your mid tower down and you come from behind to clean them up. It isn’t a great idea if you are already behind in gold.
A common mistake with trapping is to have only a few people engage from behind. This is not trapping, it is flanking. And flanking only works well if their tanks can’t retreat fast enough to get there.
Don’t ever reveal that you are flanking until after their defensive line is out of position, or you’ve just managed to get half your team killed.