When you buy a bond, you are the lender. The bond issuer is the borrower.
A is the simplest contract in finance: you give the issuer a fixed amount of money (typically $1,000), they pay you a fixed every six or twelve months for a fixed number of years, and at the end they return your original $1,000. That is it. There are three flavours, distinguished by who the borrower is. A (US Treasury, German Bund, Japanese Government Bond) is you lending money to a government — these governments can essentially always repay you, which is why they are considered the safest bonds in the world. An is you lending money to a company strong enough that is rare — Microsoft, Apple, Johnson & Johnson. A is you lending to a company with material default risk — leveraged buyouts, distressed industries, smaller companies with weak balance sheets. The lender demands more yield for two reasons: credit risk (the borrower might fail to repay — the riskier the borrower, the higher the coupon they have to offer) and interest-rate risk (the longer you lock the loan in, the more time you spend exposed to rate moves that can hurt the bond's price if you need to sell early). Lesson 3 takes interest-rate risk apart in detail; for now it's enough to know it exists alongside credit risk and that the two stack — a long bond from a risky borrower carries both.
“Bonds are loans. Yields are how much extra the riskier borrowers have to pay to get you to take their loan.”