Tar, i was also like you once and all these big words made no sense. Here is the reason time is variable. Light hits you at the exact speed whether you are going towards it or away. Technically light is supernatural...making time change around it. Thus if were going at near the speed of light on the Enterprise, a Clingon could shoot a laser from behind and still hit you at light speed. Einstein thought that crazy because every other form of matter (like a bullet), would impact the Enterpise at different speeds based on Enterprises direction. He finally considered time must be the variable...for it was the only thing left to vary, if light did not. Again...light is this crazy supernatural force in the universe that always stays constant, or invarient...which was what he originally wanted to call Relativity. Thus every moving object in the universe has its own relative time based on its speed. The faster you go, the longer time must get so that light passes by you at 186000 miles per second. It is not the 186000 that changes, it is the length of the second. if you were going 93000 miles per second your clock would still see light go by at 186000 per second...it is just that your second is twice as long as the second on earth.
So a twin could blast off very fast. His clock could read 1 year...but when he got back to earth ,his brother would have experienced 2 . I have written a book that explains it better...www.rexitivity.com...sorry you have to buy it for 9 bucks...but it answers so many things, you will be dumbfounded. Ps when they try to tell you gravity lengthens time, they(and Einstein) are incorrect. Gravity shrinks both time and space...because it can bend light and shorten its wavelength. Again, light is the supernatural force that controls time.