Basically, a bond yield is the interest rate on the bond. If I buy a one year bond of $100 with a yield of 1%, come a year I'll get $101 back. Bonds, especially government bonds, are far less risky than stocks, because you have a guaranteed return so long as the entity you're buying the bond from doesn't default/collapse. The mega-wealthy treat US government bonds as what is effectively a savings account, since if the US government collapses (:inshallah:) the wealthy have a lot more to worry about than their bonds, but in good times they get a guaranteed return/place to park their money.

When yields increase, that makes bonds more attractive to investors (because they have a higher return), so some could be re-balancing porfolios by buying more bonds, causing a sell-off. Yields can be increasing for two reasons, usually. The first is because the underlying entity issuing those bonds becomes more likely to fail, so you need a higher yield to justify holding the bond. If you think the US government is going to collapse tomorrow (:sicko-yes:), you're sure as hell not going to buy a bond from them with a 1% yield, but maybe a 10% yield would be more tempting and a better payoff if they don't collapse (:sicko-no:). The second (and more likely in this case) is because of inflationary concerns—if you think inflation is picking up, you're going to want to sell your money in bonds (because the yield is low) and park it in more inflationary assets to outpace the inflation rate. Therefore, the yield rises to entice people to buy bonds. That (I think) is what's happening here.