The steady decrease in size of atoms and ions of rare earth elements with increasing atomic number from lanthanum (Z=57) through lutetium ( Z=71).
Lanthanide contraction is a result of the poor shielding effect of the 4f electrons. Shielding effect is defined as phenomenon by which the inner shell electrons shield the outer shell electrons so that they are not affected by the nuclear charge.
Shielding effect of subshells: s>p>d>f
In lanthanides, f orbitals do shielding. Thus, it's a poor screening effect.