What does the term 'Facet' in Seaborn FacetGrid imply?
According to the facetGrid documentation :

This class maps a dataset onto multiple axes arrayed in a grid of rows and columns that correspond to levels of variables in the dataset. The plots it produces are often called “lattice”, “trellis”, or “small-multiple” graphics.

Here levels mean  « categories ». The main goal is to not display the full set of data, but multiple subsets of data that fit conditions on categorical variables.
Definition of facet found here :

facet : a particular aspect or feature of something.

In the following example, the vertical axis (column of graphs) represents the facet/feature/attribute Smoker/Not-Smoker and the horizontal axis (line of graphs) represents the attribute Time. So each graph is the conditional AND between the two attributes :

Smoker AND Lunch
Smoker AND Dinner
Not Smoker AND Lunch
Not Smoker AND Dinner


(source: pydata.org)