During a grade, you match the colours of footage taken at different times on different cameras in different lighting set ups and smooth it out so that it all matches. A grade will also correct any dodgey exposures, or even change the colour of certain objects in frame, e.g. If you want to enhance the colour of an insect to make it a more jewelled colour. A grade can also effect a style across a series so that all programmes look like part of a "family".
There are many tools to grade on, from bespoke grading suites like Nucoda Film Master to a Symphony or DS, which have grading components. You have to decide what you wish to use and what you can afford to spend.