Government and municipal bonds are low-yield, low-risk. Junk bonds are high-yield high-risk.
The part of the page you might have forgotten to read:
"The fund typically invests in equity and debt securities, including defaulted securities, with an emphasis on lower-quality debt securities. It Invests in companies in troubled or uncertain financial condition and in domestic and foreign issuers."
In other words they buy from companies on the edge of collapse, that are willing to pay very high interest for short-term money to keep the lights on. Very risky since many of those companies go bankrupt and the bonds become worthless.