The smallest three-dimensional portion of a complete crystal lattice which when repeated over and over again in different directions, produces the complete crystal lattice is called a unit cell.
This maybe compared to a wall made of bricks where the bricks is similar to the unit cell and the wall is the crystal lattice.
The unit cell can be considered as the fundamental building blocks of crystal lattice because lattice can be constructed by stacking the unit cell.
All the Best. :D