Dropped Frames

2 dropped frames after how long of playback?

Thats the amount of frames that were dropped/"skipped" instead of presented. Every dropped frame will result in a tiny lag and usually indicates that your hardware can't handle what you're trying to do. Basically, it is the most important value to keep an eye on because for best results you go as high with your settings as possible as long as you still have smooth playback without framedrops. If they stay constant while playing, your output is smooth without lags or stuttering.

Opening a file, seeking, minimizing the player etc. causes a couple of frames to drop, which is normal and doesn't really matter. But if the number is constantly increasing while watching, it usually means your hardware can't handle it.

2 Frames are nothing to worry about, i'm dropping 8 frames just by displaying the seekbar of my player, for example. What matters is that there are no framedrops while watching.