Silicone resins are solutions of branched-chain siloxanes containing residual hydroxyl groups. What is meant by "residual hydroxyl groups"?
Silicones are produced by hydrolyzing chlorosilanes( in which chlorine atoms are replaced the hydroxyl groups). These hydroxyl groups undergo condensation polymerization with removal of water molecules to form polysiloxanes or silicones.
Now, residual hydroxyl groups in your question means that some hydroxyl groups has not undergone condensation yet. Solvent is evaporated and resin is heating resulting condensation of residual hydroxyl groups to form cured resinous film.
It means there are hydroxyl groups that have not bound to (think: intermolecular forces) or reacted with something else that are still present on the siloxane chains.