A bond is basically a loan. You give me money, and I promise to pay it back by some agreed upon future time (maturity date), plus some extra to compensate you for not being able to do something else with that money in the mean time (interest). That interest amount is basically proportionate to how likely it is that I’ll actually pay you back (credit rating).

Junk bonds are basically just bonds for entities with very low credit ratings, i.e. more likely to go bankrupt before the maturity date. They carry a lot of risk, in that there’s a higher than normal chance that I’ll go bankrupt before fully paying you back. That risk is offset by higher interest rates, so if I do pay you back in full, you can make a lot more profit than other investment opportunities.

They can be used by struggling companies to get some quick cash, generally to invest into some kind of business improvements to help them stop struggling.