~Diagon Alley~

is accessible to the wizarding world, to which it is something of an economic hub, but hidden from Muggles(non magic people - people who cannot perform magic). However, Muggles are allowed access to it if they need to accompany their Muggle-born magical children.
One entrance to Diagon Alley can be reached on foot by passing through The Leaky Cauldron. The inn, invisible to Muggles, lies somewhere along the London thoroughfare Charing Cross Road. To reach Diagon Alley, one must walk up to a wall behind The Leaky Cauldron, and tap a brick, found by counting three up and two across, three times. Given the busy nature of the area, travelling to and from Diagon Alley is likely typically done by more magical means such as Apparition or by using the Floo Network.
(From Harry Potter)