Keln: You need to get & read a book or article on Special Relativity written for those with little or no background in physics & the pertinent mathematics. There might be better books for you, but the following is a good one.Relativity Visualized by Lewis Carroll EpsteinIt provides insight into both Special & General Relativity without delving into serious mathematics. You are asking too many basic questions to be answered in this forum. You are likely to get answers to individual questions without the overall context required for an understanding. Worse: There are crackpots here who do not believe in relativity. If one of them gives you answer, it is likely to be really misleading.