This is going to be short answer.
When you use (?:) it means that the group is matched but is not captured for back-referencing i.e non-capturing group. It's not stored in memory to be referenced later on.
For example:
(34)5\1
This regex means that you are looking for 34 followed by 5 and then again 34. Definitely you could write it as 34534 but sometimes the captured group is a complex pattern which you could not predict before hand.
So whatever is matched by capturing group should be appearing again.
Regex101 demo for back-referencing
Back-referencing is also used while replacement.
For Example:
([A-Z]+)[0-9]+
This regex will look for many upper case letters followed by many digits. And I wish to replace this whole pattern just by found upper case letters.
Then I would replace whole pattern by using \1 which stands for back-referencing first captured group.
Regex101 demo for replacement
If you change to (?:[A-Z]+)[0-9]+ this will no longer capture it and hence cannot be referenced back.
Regex101 demo for non-capturing group
A live answer.
