Wireframes are simple black and white layouts that outline the specific size and placement of page elements, site features, conversion areas and navigation for your website.
Hand drawn wireframes allow you to quickly iterate and test different versions of an app, while mapping out each individual screen to create your core feature.
They also allow you to focus on function and interaction before you spend too much time thinking about the surface (how the app looks).
This video gives hints and tips on creating hand drawn wireframes (4 minutes 45s)
This google video explains the importance of wireframes in the design process (7 minutes, 31s)