It has been said that there are two types of developer, the Wizard and the Scribe. Myself, I think I fall somewhere in between. The Scribe in me needs to know the definitions of words used, and also the context. Sometimes, especially when the pressure is on, I just do something that works without knowing the finer details, explaining things in a hand-wavy fashion. Not ideal. This is why, time permitting, I like to go back over code and dissect things, learning the finer points. Knowing the exact meaning of "Lexical Scope" is one of those things. Knowing the finer details helps when communicating about your code your code.
The definition given in Wikipedia is "the portion of source code in which a binding of a name with an entity applies." Let's split this into individual words.
The definition of scope in this context comes from "the extent, or area, or subject matter that is being dealt with or relevant." A more programming related definition of scope is on Wikipedia. So here we are, a Lexical Scope refers to the function's list of variables declared within it.