Peu vs. Un peu: Getting the Small Amounts Right
Peu and un peu both describe a small quantity. Peu signals a lack, not enough of something. Un peu signals a small quantity that exists, without that negative implication.
The same contrast holds when peu and un peu modify a verb directly, without de + noun:
One more distinction worth knowing: peu de works with both countable and uncountable nouns (peu d'amis, peu de temps), but un peu de only works with uncountable ones. You can say un peu de temps or un peu d'argent, but not un peu d'amis, for a countable noun like amis, quelques takes over instead (« a few friends »).
Peu importe runs on the same logic, fixed into a single expression. Literally, « it matters little, » a way of saying something matters so little it might as well not matter at all, « no matter. »