Lending is fairly straightforward.
You can either lend out xtz or tzBTC and receive an interest rate upon the amount you have lent.
You have here options to choose the amount you lend out. Then you are shown what is the APY on both xtz and tzBTC lending options (actual APY can be higher or lower than one in the example).
What do "Total Supply" and "Total Borrow" mean here? Well, the total amount of borrowing funds available and how much was actually borrowed. The higher is the ratio of utilized funds to totally available ones - the higher will be the interest rate on Lending as the bigger will be the demand for any additional assets supplied by Lenders. Note that these numbers are shown both in original currencies and USD equivalents.