The UK National Grid enables us to identify the location of a species or habitat observation accurately enough that it could, in theory, be re-visited and can be plotted on a map. The minimum level of accuracy that is preferred is 100m2, or a six-figure grid reference.
For an interactive guide to the National Grid, see here.
The ‘Butterflies for the New Millenium-Instructions for Recorders’ published on the Butterfly conservation website provides a good introduction to reading and recording grid references.
There are also a number of useful websites to help find a grid reference, including: