To sow your wild oats means to act recklessly and foolishly. Usually this phrase is used to describe a young man with prolific sexual activities. Often considered a rite of passage; a young male is expected to have a wild period before settling down.
"The wild oat (Avena fatua) is a common plant that looks like its cousin the cereal plant oat, but it is really a weed. The wild oat’s uselessness has been known since ancient times. The expression to "sow wild oats" (to sow weed seed instead of good grain) has been traced back to the Roman comic Plautus in 194 B.C. and was probably used even before him.
Sometimes young men are called "wild oats" when they are going through a foolish and reckless period in their lives.
Source: "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson, New York, 1997