Skeletal formulas are a simplified way of representing organic compounds. Instead of writing out all the carbon and hydrogen atoms, they are implied by corners, also called ‘vertices’, in the structure. See for example Figure 1. Only carbon and hydrogen can be omitted this way.
Figure 1. The organic molecule benzene with all atoms drawn out (Structure A), and the simplified skeletal structure (Structure B), which is commonly used. The skeletal structure of salicylic acid with molecular formula C7H6O3(Structure C)
Skeletal structures are a great way to more clearly show important aspects like functional groups of an organic compound, see Figure 2 for the skeletal structure of salicylic acid. A combination of the two ways of drawing the structure can be used as well.