Fixed interest is the generic name for debt securities issued by companies and governments.  Alternative names for these are Corporate Bonds and Government Bonds.  In the UK, Government Bonds are also known as Gilts or Gilt-Edged Stock.

If a company or government wishes to raise some funds, they can issue Bonds offering a fixed rate of interest for a certain period of time.  At the end of the term, the capital amount is repaid. 

Normally, Bonds are rated according to the risk of default on the debt by the issuing company or government.  The higher the risk of default, the higher the rate of interest offered.

Bonds can be traded by individual investors and investment companies, and their value in the market can be, and usually is, different to the face value.

The value can fall as well as rise, so fixed interest investment carries a degree of risk - there is a chance you might not get your full investment back - but this is generally lower risk than investment in shares.

Fixed interest can be useful for providing a higher than average income, with the opportunity for some capital growth.  However, over the long term, you are likely to achieve a lower overall return than investing in shares.