Buffer Retainer Spring

The Buffer Retainer Spring is used to keep the buffer retainer in place to keep the buffer and buffer spring in place.  Here is a picture of one:

Buffer_Retainer_SpringYou can purchase one as part of a Lower Parts Kit, or individually here, or here, or here

Here you can see how it is installed in the lower with the buffer retainer, the buffer tube actually will keep this in place.

AR15 buffer detent and spring installation