At the point of a scene change, a transition lets the reader know where they are, when they are, and who they are with. These sections are necessary because the reader can become disoriented when moved from one scene to another or into or out of a flashback. Transitions are especially important when moving from one point of view character to another. Without those clear transitions, the reader becomes lost. Who are we with now? Where and when are we now? Imagine waking up to find yourself in a spaceship surrounded by little green men and women. You'd be asking, how the heck did I get here?
 
So each time you start a story or new chapter, move to a new time and location, or switch to a new POV character, you want to give the reader a transition. Here you let the reader know where we are, what time it is, and whose POV we are now in, and touch on the situation the character now finds themselves in. You can do that through description, exposition or a short scene. Here's an example of a scene transition at the opening of Chapter Two in my literary novel The Spawning Grounds:
 
"Stew's border collie, Abby, stood on alert, staring at something in the water. His mare watched the river with the dog, her ears cocked forward, and it was then, as Stew looked to see what had caught his animals' attention, that he saw the boy standing on the water."
 
With this short scene transition, the reader knows they have moved to a new character (Stew) and to a new location (the river). But this transition also draws the reader in with something of a mystery. Who is the boy, and why is he standing on the water?

In learning to write transitions, the best teacher, as always, is an example. Study scene transitions in novels you admire. Was the transition as long as a paragraph, or as short as one or two words? Did the author place the transition at the start of a chapter? Did they use a space to make it clear to the reader that they were moving to a new time or place? What indication of the passage of time did they use? Did they use a date, for example, or a day of the week, or indicate it was morning or evening? Or did they describe the location to show that change in time? How did they use descriptions of place to tell the reader they were in a new location? If there was a shift in point of view, how did the author make it clear the reader was moving to a new POV? For example, did the author start a new chapter to move to a new POV? Or name the new POV character off the top? Pay special attention to how the author used transitions to set the tone or mood of the scene change. Did the author use a different style of words or description to indicate to the reader that there was a shift in mood or tone? Was the transition used to offer the reader a breather after a scene of high action, for pacing? Was the transition used to pass over a time period or some events? Why?

Want to know more? Here's some Tips on Writing Scene and Chapter Transitions.