Ah, thank you for asking! This is covered in the GTFS page of the wiki, here:
https://wiki.openstreetmap.org/wiki/GTFS
Given your question, I don't know how much of this you're already familiar with, so I'll give as much information as I can.
Obviously, the "gtfs" portion of the key specifies that this refers to public transit GTFS data. The "shape_id" specifies that the value "shp-1-04" is the unique identifier for a shape file in the GTFS data corresponding to this specific variant of the route. And the final "US-WA-PT" is a unique code referring specifically to the GTFS data for the Pierce Transit network, found here
https://wiki.openstreetmap.org/wiki/List_of_GTFS_feeds#Washington
so that anyone who wants to link OSM and GTFS data for improved public transit routing knows where to obtain the data.
Please let me know if you have any other questions!