Viscocity is a liquid resistance to flowing. Thicker liquids tend to have a higher viscocity.
Let's take honey as an example. The bonds in honey are hydrogen and oxygen form hydrogen bonding, which are one of the strongest bonds, and this causes strong cohesion forces between them and it causes the flow of honey to be slow/viscous.
Cohesion is the attraction in a molecule of liquid to each other due to intermolecular forces.