They are structures used to help scientists estimate the amount of bonds required for an atom and also see what the molecule will look like.

The first rule to drawing a Lewis structure is knowing the different bonds and the significance of each symbol. The dot represents a lone pair and the line represents a shared pair. For each line it is given a value of two bonds(since it's shared).The lone pairs don't have any bonds but are given to the compound in order to follow the needs of each element. Each element requires at least 8 electrons; this is called the octet rule. However, Hydrogen is the only element to be satisfied when it recieves two so, it defies the octet rule.

To predict the molecular geometry of a Lewis Structure you need to look at the amount of lone pairs it contains. When the Lewis structure has 1 lone pair it will be considered "bent", where the pairs have 120 degreees in between. If the Lewis structure has three bonds with no lone pairs it is called the trigonal planar. If there are four bonds you can have a tetrahedral. Depending on whether the structure has lone pairs or not it'll change the name and appearance of the structure.