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
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).