Hi there!
A neutral metal sphere is comprised of both ELECTRONS and PROTONS.
Recall that electrons have a NEGATIVE (-) charge, and protons have a POSITIVE (+) charge.
Opposite charges attract, and like charges repel.
In this instance, the positively-charged rod will cause the electrons in the neutral metal sphere to move towards the side facing the rod (opposite charges attract), which causes the two to be attracted to one another.