A Bond is an investment instrument. Generally, a bond is a fixed income instrument in which you (the investor) will get a fixed amount of interest (coupon).
Bonds work like a loan. Generally, loan means you will borrow money from a Bank. You have to pay back the loan amount along with the interest in the form of EMIs (monthly installments) to the Bank within the loan term.
But, in Bonds, the loan system is a bit opposite where you (investor) are giving money to the Bond issuer. So, the Bond issuer will be responsible for paying you back the amount along with the interest within the bond term.
Generally, Bonds are issued by the Government and other Organizations to raise funds for a development project.
For example, Government of India can issue a bond to raise funds for building a national highway, dam or any other infrastructure project for the development of the country.