noun, verbal noun
Something has drifted to the point where it has arrived. Where do drifting things arrive? They arrive on shore (as they drift on top of water). That makes this word drifting ashore.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
Imagine a wounded baby whale drifting ashore onto a sandy beach.