Above is what you see when you go into SYNTH EDIT for synth1 or synth2. In this post, we focus on the VCO controls.
For both oscillators, you can choose the type of wave signal: triangle, sawtooth, square (aka pulse), noise. The default is sawtooth which is kind of a whole-purpose base signal. The balance knob enables you to mix the two oscillators. The VCO 2 pitch knob lets you control the pitch of the second oscillator relative to the first one. You usually want the second oscillator to be at a higher pitch if you intend to sync them.
Oscillator sync (VCO SYNC) is an important tool since it will sync (synchronize) the second oscillator (usually called the slave oscillator) to the first oscillator (the master oscillator). If the second oscillator is modulated and its pitch changes (for example, with a low frequency filter or LFO, called a modulation generator or MG on KORG synths), oscillator sync forces the second oscillator to start over every time the first oscillator starts a new cycle, locking its base frequency (to be always the same as the master's). Syncing also prevents a phenomenon called beating, which usually happens when superimposing two oscillators operating at different frequencies. It's most noticeable when the second oscillator is at a slightly higher pitch than the first one (the difference between the two frequencies gives you the frequency of the beat). Practical note: when you use oscillator sync, you may want to mix down the master to actually hear the oscillator sync effect.