It's a drain hole, or more specifically, a "water drainage hole." Reference: SA-SIP-7790476 Rifle, U.S. 7.62-MM, M14 National Match. Department of the Army Springfield Armory January 1966: page 62.
Why is it there? From M14 Rifle History and Development Fourth Edition:
"During the rain test of a T44, the barrel channel in the stock became flooded. So, Mr. Savioli solved the problem by designing the 5/16 " diameter hole into the bottom of the stock just forward of the sling swivel."
Reference: Humphreville, Mark. Telephone interview. March 18, 2007. Mr. Humphreville was a co-worker and friend of Guilio Savioli in the 1970s at the General Thomas J Rodman Laboratory located at Rock Island Arsenal. After Springfield Armory closed in 1968, Mr. Savioli worked at the Rodman Laboratory at Rock Island Arsenal and then at Picatinny Arsenal. He retired with thirty years of government service and at least eleven U. S. Patents to his credit. Mr. Savioli passed away in 2009.