There was this dude traveling at 90% of the speed of light...?

I was reading "Relativity: An Introduction for Younger Readers." One of the chapters was about the twin paradox. Some dude named Al went on a spaceship and traveled 10 lightyears. It only felt like 36 days had ped for him and 10 years had ped for his twin Ike . I don't understand why it only felt like 36 days,for Al, which means I don't understand relativity. Can someone explain this to me?