"Simplemente" is an adverb. Most adverbs in Spanish have that ending. It doesn't really mean anything except, I suppose, that the word is likely an adverb. I'm not aware of any other part of speech that ends in "-mente".
I'm pretty sure it's the same as how we add "-y" to the ends of adjectives to form adverbs. In English, "simple" becomes "simply", and in Spanish, "simple" becomes "simplemente".