Hi Pavan,
Endcap cells are used for terminating rows as the end of diffusion. These cells are used to fullfill NWELL Antenna requirement. (you can cehck the corresponding rule in your design manual) .
In the design or say few technology normally has special requirements such as latchup rules. In order to make layout DRC and electrically latchup compliant, endcap cells are created specifically for the reason of eliminating these errors.
END CAp cells normally contain only tapping layers N+ and P+, and there layers does not interfere with routing, however, the endcap cell will block any other cells from being placed at that location.