could anyone explain this to me ... Leaflet

Leaflet displays a map by displaying a series of tiles across the page. It needs to know where to get these tiles from and how the tiles are stored in a directory structure. The statement you quote shows leaflet how the OSM tiles are structured. If you used tiles from another source the structure would be different.

Leaflet has fairly good documentation, the part describing how the tiles are structured is here: http://leaflet.cloudmade.com/reference.html#tilelayer

That line looks like the placeholder string for the url to the tiles that make up the standard layer. Leaflet will replace all the strings in curly braces '{}' to get the url for the tile it wants.

For instance http://b.tile.openstreetmap.org/8/127/85.png will give you a nice tile covering the south of england.

The tile layer definition has z, x, and y in it. They're replaced with actual values whenever Leaflet needs to fetch a tile. If you've got a working map, but it's centered on the wrong, you probably just need to tell Leaflet where to centre the map.

If you look at this example, you'll see that it contains:

map.setView(new L.LatLng(51.3, 0.7),9);

It's that function call that tells Leaflet where to centre the map and at what zoom level. If you replace "51.3" with the latitude you want, "0.7" with the longitude, and 9 with the zoom level, you'll get a map centred wherever you want it.