This is the best and most simple explanation I found on what is the problem of scalation that Bitcoin is facing.
It explains what is Segregated Witness (Segwit), the Lightning Network (LN), why increasing or even decreasing the block size does not fix the problem (it decreases security).
https://hackernoon.com/beginners-guide-to-bitcoin-s-scalability-debate-66060f3799e5
By Kevin Huynh and Steven Chen
I find it a great article because weights the different options without pushing an agenda and highlights the need for further improvements, instead of bashing one or another option based on what coin the authors are invested in. Very good article IMHO.