Q286. When using Leaflet to create a webmap, we use code like this:

var cloudmade = L.tileLayer('http://{s}.tile.cloudmade.com/{key}/{styleId}/256/{z}/{x}/{y}.png', {
            attribution: 'Map data © 2011 OpenStreetMap contributors, Imagery © 2011 CloudMade',
            key: 'BC9A493B41014CAABB98F0471D759707',
            styleId: 22677
        })

to create the "attribution" on the map. What does this refer to? Why do we need to do it?