It is comprised of many elements that can differ in dimensions (1D, 2D, 3D) and shape (triangle, quadrangle, tetrahedron...).
The size or the number of the elements in the mesh is in proportion to the quality of results. This means: the smaller you choose the elements, the closer you usually get to the ideal results. This indirect proportional relation also exists between the element size and the calculation duration. So in everyday engineering practice, the element size is sought that ensures sufficient accuracy and acceptable calculation time at the same time.