A covering is required for the floor of the cage to provide a comfortable surface for the rat and also to soak up the urine. The most common type of floor covering available is woodshavings - fine sawdust should be avoided as this can cause irritation to the eyes.
Cedar woodshavings (usually distinguished by a red tint) should not be used as the phenols they contain can cause severe irritations in rats. Although Pine is a softwood Pine shavings cause less problems than Cedar and kiln dried Pine can be used without problems.
Any form of scented shavings should be avoided as these can also cause irritation to rats. Corn cob bedding can become mouldy when wet and can also swell if ingested and so is not ideal.
Woodshavings from hardwoods such as Aspen or small animal litter made from wood pulp (such as Carefresh) are the safest form of floor covering to use and these are available to buy online from a variety of pet stores.