Mortar or grout is a cement like product that is installed between the joints of the bricks in order to help keep them together.
Merriam-Webster Online Dictionarymortar
a sturdy vessel in which material is pounded or rubbed with a pestle
a) a muzzle-loading cannon having a tube short in relation to its caliber that is used to throw projectiles at high angles
b) any of several similar firing devices
a plastic building material (as a mixture of cement, lime, or gypsum plaster with sand and water) that hardens and is used in masonry or plastering
to plaster or make fast with mortar