A transit of Mercury takes place when Mercury passes directly between the Earth and the Sun. During the transit Mercury appears as a very small black dot travelling across the Sun. This cannot be seen with the unaided eye. For information on how to view the transit check out our page on safely viewing the transit.
Transits are less common than solar eclipses (when part or all of the Sun is blocked by the Moon), and there will only be 14 this century (two of these have already happened and another one will not be visible from the UK). To find out more about why we get transits check out this leaflet from the Society for Popular Astronomy and watch this great video from Europlanet.