What do you mean by torsional strain?

(N/A) Alkanes can have an infinite number of conformations by rotation around $C-C$ single bonds. However,it should be noted that rotation around a $C-C$ single bond is not completely free. It is hindered by a small energy barrier of $1-20 \ kJ \ mol^{-1}$ due to weak repulsive interactions between the adjacent bonds. Such a type of repulsive interaction is called torsional strain.