Something that's plenteous is abundant — there's plenty of it. If the spaghetti at dinner is plenteous, you won't think twice about having seconds, or even thirds.
You can use the adjectives plenteous and plentiful interchangeably, although plenteous is the more literary and poetic of the two words. A farmer might say happily, "The corn harvest sure was plenteous this year!" or you could say, "My friends are so plenteous, I'm having trouble choosing just three to invite to my pool party this weekend." Plenteous was originally plentivous, from the Old French plentiveus, "fertile," from plentee, "abundance."