What is this simple irule doing?

This will insert a header called "HttpsIndicatorHeader" with a value of "True"

Further reading: https://devcentral.f5.com/wiki/iRules.http__header.ashx

If you want your iRules to execute in a specific order, use priority as descried here https://devcentral.f5.com/wiki/iRules.priority.ashx

As to why you need to insert a header, that's something that can be only answered with knowledge about your environment or application