While ceramic tile is a durable, low maintenance material, the grout between the tiles is not. Unless stain-proof grout was used, the grout will need to be sealed. Allow the grout in new floors to cure for 30 days before sealing.
If the grout is stained or dirty, use a mild bleach solution and scrub brush to clean it, then allow the floor to dry completely before sealing. If the sealer doesn’t come with an applicator, buy a separate applicator bottle with interchangeable tips.
If the grout has become stained and won’t come clean, use a special grout stain pen that both stains and seals it at the same time.