Grout is a material made from sand, water and cement, and is installed between tiles where tiles meet other surfaces. Grout helps to fill gaps and create a water resistant seal. It is common to find cracked grout at locations where the tiled surfaces meet other surfaces, such as at floors or countertops. Grout is not an elastic material, and will not stretch when building materials expand and contract, or when building structures settle. Therefore, when movement occurs grout can crack.
Since grout cracks are very common, building material manufactures have created elastic sealants than can match the surrounding grout, and actually look almost identical to a grout surface. Many of these sealants have sand added to them to match the sometimes coarse surface texture of grout, so when the sealant is installed, there is hardly a noticeable difference between the grout and sealant.
Cracked grout lines at countertops and floors are a common defect in newly built homes, and cracked grout repairs is one of the most common warranty claims for new construction homes. It is also common to find grout cracks inside tiled shower enclosures. These cracks when exposed to excess moisture can allow water to leak into walls and floor, and could cause substantial damage over time if not addressed. Best building practices calls for sealant to be installed at shower enclosure corners and transitions to prevent cracking and water intrusion issues.
Check out the video below on more information on how to repair grout cracks.