Base Stats can be thought of as the "lowest" stats or what is built on to get the final stats. They come in two varieties.
Character Base Stats
These are the stats that characters have when they first appear on the map, also known as a character's starting stats.
Generic enemy or NPC characters can have variable base stats since they are auto-levelled using the class growth rates.
Class Base Stats
These are the "lowest" stats that a particular class can have (although it's possible to go lower if a character has negative stat additions (such as Gwendolyn's Constitution). They appear in the class roll that some games feature as their opening demo.