The explanation is present in the link I had provided earlier. I will add it once again for your reference.

http://www.sqlservercentral.com/articles/comma+separated+list/71700/

There is another article which contains a few more methods to achieve the same result

You can check those as well and use whichever you prefer

https://www.simple-talk.com/sql/t-sql-programming/concatenating-row-values-in-transact-sql/

Edit:Added one more link to an article