Most simple example to explain the concept the development. Take your first finger and grab it in the palm of your other hand. Now pull the finger applying as much force as you can. At a certain limit the finder will slip and the contact between the finger and the palm will break. But suppose if my finger is too long or the grip between the finger and the palm is immense such that if you pull your finger then the only way to get your other hand free is breaking up your finger. That is the base concept of development length.

The bond between steel and concrete is somewhat similar. We cannot have a continuous bar of steel throughout the structure. There are always connections, joints and splices. If we do not provide development length, then at the location of these critical areas the structure will fail easily. But if we provide sufficient amount of length to give a continuity to the strength of the structure then the strength will be same everywhere.

As shown in the picture above, I have chosen an arbitrary size of bar and displayed how the grip between the two surface is gained. Now if you sum up the grip then you can see that the bond strength increases from a zero value to a certain value X. Now we wish to increase the value of X equal to the strength of the steel used in that structure. So whenever the structure is loaded the only possibility of failure is by the rupture of steel and not anything else like failure of bond. So that is why we provide the development length. The minimum amount of length required by the bar to develop the full strength of the bond.

This development length depends on various parameters.
1. Grade of steel
2. Grade of concrete
3. Diameter of steel bar
4. Type of steel bar and so on.