bandwidth of the microstrip antenna........

Not all such antennas have a narrow bandwidth. Only resonant ones have. Many designs use additional elements to expand the bandwidth.

what is means resonant?

With your knowledge at such zero level, I do not think I could explain in short what it means.
Please read a basic textbook on electronics and electromagnetism basics. Without the basic knowledge you could not understand most of the talk at Edaboard RF, MIcrowave & Optics

ok,,what is the effect if has smaller BW to the performance of antenna?

Because its has relation between a bandwidth and a Q factor of the microstrip antenna.
Narrow bandwidth is a high Q factor. Good luck

Because microstrip antenna has a greater complexity (to make more effective use of the enclosing volume), larger size (leads to a larger enclosing sphere), or the introduction of loss (reduces the radiation efficiency) and it only comes when the "electrically thin grounded substrate" retained (which is the caused of the narrow bandwidth).