bonds are basically IOUs from a government or company.
For example
Say they borrow £1000 from you, they will promise to pay you back in a certain way. The two methods are:
110 pounds for the next 10 months = 10% or
100 pounds for 10 months then a 100 extra at the end, they are taxed differently...
You can don't "lose" money on holding the bond for the whole life cycle and you will know what you get in return as long as that government/company don't file for bankruptcy, or some other form of debt restructuring.
Say you brought a bond that gives 5% rate of return, then the interest rates changes so that it's lower and the new bonds are only offfering 4%... in theory you can sell your bond to someone else for 4.5% (as an example) and pocket the money now to spend/re-invest rather than have to wait till the whole life cycle ends.
but if the interest rates goes up to 6% for new bonds, you may have to sell your bonds at much lower rate to cash in quickly.
the other thing is inflation, say you brought a bond with the rate of return for 3% and banks are offering 5%, then you're basically lossing 2% per year.
The rate of return and lenght matters, some companies will offer a high rate of return if they don't have the "credit" status to borrow money at lower rates.
Government bonds have lower rates as it's basically the government and they will get you your money.
There are times were a company like vodafone, who have brought back some of their bonds at higher rate to get rid of the debt.
Then there's the whole FX trading, where you by another countries bonds... say if the pound is stong vs the dollar at 1 pound vs 1.30 dollar, you buy 1300 dollars of bonds..you can wait till the dollar is stronger and sell the bond at some loss but make the profit on the exchange rate.