You place it by the wall or on a tree to let the little people come in and out! Mine is by the fireplace in the bedroom because I know boggarts like to live in such nooks.
Here's a link to a site that sells them! Mine's quite plain though, and it came in a choice of sizes, so I'm not quite sure these are exactly the same doors. I found mine in one of Whitby's many weird and wonderful little shops.
In fact... Here are some more Faerie doors!
http://www.iconsculpt.com/q/page/e/s...roduct_id/e/97
http://www.beneath.co.uk/fd.html
And...
this one is exactly the same kind as mine!