Noradrenaline/norepinephrine is an aromatic organic compound containing a benzene ring (the hexagon). Essentially, the vertices in the diagram are carbon atoms and the number of lines represents the covalent bond order; 1 line (eg connecting an OH to the benzene ring) means a single covalent bond. Any 'spare' bonds are where a C-H bond is. So it can be rewritten like this:
HOWEVER, when it comes to a benzene ring (the hexagon), you do not simply have alternating single and double bonds. In fact you have what is known as a 'resonance structure', where the electrons/bond is shared around the entire ring. One way of thinking of it is that there are 1.5 bonds between each carbon atom in the ring. Another way of thinking of it is that pi-orbitals from each carbon atom merge to form a ring combined orbital structure, so instead of existing in that figure-8 type area around one carbon nucleus, those 6 electrons can be in a donut shaped area above and below the carbon nuclei:
This is why you often see a hexagon with a circle inside it to represent a benzene ring as it is less misleading than the alternate single and double bonds. In that case, you would redraw the noradrenaline diagram like this:
The advantage, though, of having the alternate double and single bonds is in drawing diagrams when working out reactions.