Tile grout is the substance that is used to install tiles. There are all types of grout available depending on what you need it for. You will most probably have a grout mixture that you will have to make to yourself.
Get this grout mixture together just before you are ready to install the tiles. Be sure to follow the instructions that come with the grout as accurate measurements will affect how long it takes for the grout to set as well as how effective it is.