First of all, Wikipedia should help you: https://en.wikipedia.org/wiki/Abstraction_(computer_science
From myself I can explain it by example.
I don't know you. I never met you. I don't know where do you live. I don't know whether you're a man or a woman. I don't even know whether you're a human being or a computer bot.
But I'm writing this message to you now.
All I need in order to do this is to abstract myself away
from who you really are and deal with your abstraction
in my head and in this website.
This abstraction in my head knows how to read my messages. That's all I need to know about you.
Thus, abstraction is a simplification of real world things in order to make more digestible