In chemistry, what does the double line mean on a skeletal formula? [photo]?

In such "stick-form structural formula", a single line represents a single covalent bond, a double line represents a double covalent bond, while each "vertex" or "terminal point of a line" represent a carbon atom, i.e.

"-" represents a C-C single bond
"=" represents a C=C double bond
"N-" represents a N-C single bond
"N=" represents a N=C double bond

The structure of the compound is drawn again and put in :
https://c2.staticflickr.com/8/7329/27300979001_da9f425661_o.png

With organic molecules, when drawn as a structure, it is understood that there are carbon atoms at the intersection of each bond (unless another element is put there instead) and that there are hydrogens attached to those carbons in the number that is needed to make four bonds for each carbon.

Double bonds are shown by two lines together rather than one. A triple bond is shown by three lines.

When an N is shoved into a position normally occupied by no letter at all (meaning normally it would be carbon), it means that there is a nitrogen atom at the location instead of a carbon atom. Nitrogen can only form 3 bonds instead of four like carbon, so you can see in that diagram that one of the Ns has no H, because it has three bonds connecting to Carbons in the ring (one double, and one single bond=3 bonds), but at the other two locations, there is NH and NH2, to show that the nitrogens are not totally bonded to carbon.

Every carbon has to have 4 bonds, every nitrogen has to have 3 bonds, oxygen has to have 2 bonds, and so on, with the number of bonds imposed by the element and its electron structure. If only two bonds are shown for a (not-shown) carbon, then there must be two H atoms attached to it to satisfy the bond requirements. They simply are not shown because it has to be that way.

For a chain of carbon atoms, the standard notation is a zig-zag pattern with single lines, and the pattern means there is a carbon at each end and a carbon at each intersection. If you were shown an upside-down V pattern, it would represent CH3-CH2-CH3. If you see an NH2 at the end instead, the compound would be CH3-CH2-NH2.

The types of bonds that occur are what define different "functional groups", which means that things like amines (-NH2) and alcohols (-OH) have a special chemical behavior, and their existence in a structure says a lot about how the compound will behave (or how it was made).