The preposition behind is commonly used to express someone or something located at the back or to the far side of another thing. An important point to understand is that the person or object in question does not need to be visible in order to be behind - it can be visible, partially hidden or totally hidden.
For example, in the following picture a mobile phone is placed on the opposite side, or at the back of a cup of coffee. We cannot see the phone, as the cup is concealing it, but we know it is there. If we replace the cup of coffee with a glass of water, the phone becomes visible as we can see it through the glass. In both cases, what does not change is that the phone is behind the two objects:
"I looked behind me and saw the train arriving."
"His house is just behind mine."
"She dropped the remote behind the sofa."
"The Sun was hidden behind the clouds."
This final example proves another important concept to understand, which is that behind does not always mean directly behind something else. The Sun is in fact millions of miles away from the clouds; however, from our perspective, it can still be described as located on the far side of, and concealed by the clouds. Therefore behind is the correct preposition to use.
Behind is also commonly used when referring to someone or something further back or in a less advanced position than other members of a group. This definition is flexible and can be applied to numerous situations. For example, in a classroom or office, someone might be described as being behind their peers in terms of ability or productivity. An individual or group might be described as being behind in terms of social status, and in competition, competitors can be described as being behind their rivals in a league table or race.
In the above picture, the car at the back is behind the rest of the group. However, it would also be accurate to look at the picture in another way and say that the yellow, blue, and red cars are behind green, the leader:
"Jimmy was behind the rest of the class with his homework."
"Her football team were behind the current champions in the league table."
"Due to his background, he was behind the other candidates in the pecking order."
"The company were lagging behind their rivals in sales."