What does it mean to rationalise the denominator?

To rationalise the denominator is to remove all square root signs from the bottom of the fraction. An example would be if you had the fraction 1/sqrt(5). To rationalise this fraction, you would multiply the numerator and denominator by sqrt(5) to get sqrt(5)/5. This is rationalised as the denominator does not contain a square root sign even though the numerator does.