Bonds are sold at a particular interest rate. The price of the bond is calculated, against a base of $1,000,000 each, so that the interest is in the purchase price.
When I last bought bonds, the yield was 4% and change, about 3.89% after the bank's fees. So the price of the bond was calculated to give me back $1,000,000 per bond at 3.89%, after the period of 30 days (30 years in game time) had passed. I paid something like $680,000 per bond.(Don't hold me to that, as I really don't remember what I actually paid.) At the end of that 30 days, I will receive $1,000,000 per bond. I bought 200 bonds, so that will be $200,000,000.
There is a rate calculator on the Purchase part of the Bonds screen. It will tell you what you have to pay at the current rate to receive the Par Value ($1,000,000) at the end of the term. If you wish to cash in earlier, ti also keeps a running tab of where you are and what you will receive if you do cash in now. The rate will be adjusted, but you should never lose money.
You will pay taxes on bond profits.
Bottom line is, bonds are a useful way to keep earning on your excess cash. It ties it up, in a loose sort of way, for the duration of the bond. I would try to buy when rates are over 2%, which I consider piss poor, but seems to be reasonable in this milieu. This thread was started because bonds yielded over 4% at the time, and I bought a bunch of them. You can buy them whenever you wish, and each 'basket' that you buy is treated like its own investment. I once had four rolling baskets going of $10,000,000 each. I suspect I should have a lot more going now, as I have a lot of cash, but I am stubborn about yields, so I don't.
I would be careful if you don't really have "excess" cash. You will need that cash at some point, and you should hold enough to handle occasional market purchases, material purchases, etc... But if you truly have a bunch of cash sitting around, bonds are a good idea.