The Torah tells us that Hashem led the Jews through the desert by day with a pillar of cloud and by night with a pillar of fire.He did not move the cloud from its position by day nor the fire by night.
Rashi explains that this means there was an overlap of a few moments. By day Hashem brought the pillar of cloud and then removed the fire and by night He brought the fire and then removed the cloud.
Why is this such an important fact that it needs to be mentioned in the passuk? What is the lesson taught?
I think the idea is that the jews were not left unportected even for a moment. Here they are leaving the "comforts" of home, going into the unknown of the desert with the possiblity of war and who knows what other troubles would arise. The Jews naturally would be nervous. Are we doing the right thing? How do we know Hashem wil protect us? And all sorts of similar questions.
The passuk is telling us that Hashem was calming the fears of the Jews. He wants us to know that we are never left unprotected even for a moment. Sometimes it may appear as though Hashem has abandoned us and we have troubles of various sorts, but we have to be aware that we just do not recognize His presence and do not understand His plan. Hashem does not abandon us.
Hashem created an overlap in the desert just so the Jews would be aware that they were protected at all times. Normally He does not go so far as to let us know that, but those were unusual times and He chose to let them know. The lesson for us is that even when we are not told specifically, we should still be aware that it is still true.