Lattice multiplication is a method for multiplying multi-digit numbers by drawing a grid and placing the numbers to be multiplied along the sides. For each box in the grid, the number above is multiplied by the number to the right and the products are written in the boxes. The numbers are then added along the diagonals, with carries as needed, and the final sum is the answer. The document provides examples of multiplying a 2-digit by 1-digit number and a 2-digit by 2-digit number using the lattice method.