Explain KITE Publisher and Connect?

Explain KITE Publisher and Connect?

Is that useful only for Programmers? or for common user as well?

Useful only for programmers trying to build trading front ends, or for websites/bloggers/etc to integrate buy/sell button within their interface.

Check this: http://kite.trade/

The API doc: https://kite.trade/docs/connect/v1/#introduction