What is Development Length?
It is the minimum length of the bar which must be embedded in concrete beyond any section to develop its full strength.
This is also called as an anchorage length in case of axial tension or axial compression and development length in case of flexural tension or flexural compression.
It is provided at column beam joint or column footing joint. Here you can see in the picture this is development length for beam-column joint.
The main bar in the beam which is extended from here to here (as shown in the image below) to safely transfer stresses from beam to the column and this portion of length is called as development length.