A Lewis dot structure (named for American chemist G.N. Lewis) is a representation the basic chemical structure of a covalent or polar molecule with letters (representing atoms) and dots (representing valence electrons).
H H
.. : ..
H : O : : O : O : H : C : H
.. .. .. ..
H
water oxygen methane
There are as many total dots in a Lewis dot structure as there are valence electrons in the atoms represented. Light elements (hydrogen, helium, lithium, and beryllium) want to be next to two dots; other elements generally want eight dots (except silicon and sulfur, which can often take 10 or more dots). Atoms tend to share pairs of electrons.