The SEC yield is a standard yield calculation developed by the Securities and Exchange Commission (SEC) that allows for fairer comparisons of bond funds. It is based on the most recent 30-day period covered by the fund's filings with the SEC. The yield figure reflects the dividends and interest earned during the period, after the deduction of the fund's expenses. This is also referred to as the "standardized yield."
A good source for definitions like this:
SEC Yield Definition | Investopedia
I'm looking at information on bond funds on the Vanguard site. For example the Long Term Bond Index Fund (VBLTX) states that the SEC yield is 3.85%.
What does this number reflect?