What does high viscosity mean?

High viscosity (highly viscous) means lower flow ability; low viscosity (of low viscosity) means better flow ability,

Example:

• Honey = highly viscous
• Water = of low viscosity

Liquids with a high viscosity spread more slowly on the surface of the adherend (joining part). 

Are all hot-melt adhesives highly viscous?

The information on the viscosity of a hot-melt adhesive is part of the technical data, which is shown in the processing data sheet of the product.  

In the case of hot-melt adhesives, the lower the test temperature, the more viscous the adhesive.