A transit is where 2 objects are in line. It could be that they are marked navigation transits or it may be 2 marks you have decided will take you to where you want to be. As an example, divers often use 2 sets of transits or more to place their shot line on the wreck they are looking for rather than dive on "HMS Vicinity".
The following images are examples of transits.
Leading lights in one and a prominent feature with a light in the other.
The following images are examples of on track, off track to port or stbd.
When looking at a transit, the biggest clue on which way to turn to regain track is to look at the bottom transit.
If the bottom transit is to the left of the top transit go left (if it is to the right then go right) until just before regaining the transit. When you have straightened up you should find yourself back on track.