To add to the above answers, grout is like the cement used to stick the tiles in place(and fill in the cracks between). It comes in various types, but any serious store can help you with which kind. You can also get colorants to make it different colors.

this is grout the tiles are laid with a slight gap between them when the floor is fully tiled and the adhesive set a grout is then used to fill all the gaps around the tiles and you can gat different colours yours is grey

those are called "grout lines" - and you set the width of grout lines depending on the sort of tile. Floor tiles like these are often set at 1/8 to 3/16" wide and filled with "sanded" grout. Tiles in showers were often set extremely tight - maybe 1/16" and filled with "unsanded" grout.

Like the grayish part I circled in between tiles. Trying to buy some s