Think of the moment something stops feeling tired. A wall that was grey gets a coat of white paint. A closed room gets its windows opened. A half-cold cup gets more poured into it. The air turns cool after rain. In every case the thing itself is not replaced — it is the same wall, the same room, the same cup, the same afternoon — and one small change makes it seem new again. Freshen is fresh with -en on the end, and -en simply means make it that way, or become that way.