1. Chess bot can force engine to play in two differnet ways: by depth or by time. When play by depth method is used, bot will ask the engine to calculate the moves strictly to a certain depth.
When playing by time, engine will decide on itself on how much time to spend on thinking about the current move. Choose "play by time" method, when you need to achive the strongest play.
2.Engine depth parameter specifies on how deep should engine calculate positions. Higher the depth = slower the moves = stronger the play. For example, at depth = 1, chess engine will play very fast, but very weakly and will make mistakes often. And at depth = 20 engine will crush grandmaster easily, but moves will be much slower.
3. "Floating depth" feature will allow you to use different engine depth in different parts of the game. For example, you can set depth = 20 on first 10 moves, depth = 18 on moves from 10 to 20 and depth = 14 after that, etc.
4. If you are choose "Play by time" method, you should set time control, so engine will know how long it can think on the moves. For example for classic bullet game, you should set 1 min + 0 sec increment.