Think of bond like the more introverted brother of stocks. It is a lot less volatile and returns are lower but reliable and good hedge for stocks.
A bond is a loan issued by government or to anyone who wish to take up the loan. By purchasing a bond you are a borrower to the institution and you get paid interest in the form of coupons which are paid out regularly once or twice a year.
The period is usually about 10 years or more depending on the issuer. You may choose to buy the bond at the beginning or in the middle. You can also sell it anytime so you are not tied up for the whole duration.
You can think of it like a fixed deposit account that pays you interest about 3 to 5 %.
Here are some of the SGS Bonds listed in SGX if you are keen to balance your portfolio with Bonds.