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
    ..     ..   ..        ..
  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.