This usually refers to a second mortgage, and is tied to an adjustable rate. The most common lines of credit follow the Prime Rate.
A line of credit differs from a standard amortized first mortgage in that the money can be taken and paid back as needed from the equity in the home. These lines are often available with a check card or check book so you can take out cash conveniently.
This can be an excellent resource for financing home improvement projects or keeping easy access to an emergency fund for your family. Usually, the interest payments are also tax-deductible. Be sure to consult with your own tax advisor.