Lạc was an ancient name of Vietnam's history. It was just a name as long as I know.
Lạc Việt was a tribe. Âu Lạc was a state. Lạc Long Quân was a legendary lord.
You can consult the definitions of these expressions on Wikipedia by searching "Âu Lạc", "Lạc Việt" and "Lạc Long Quân".
Do not confuse "Lạc" with "lạc". Nowadays we rarely use "Lạc" in daily life, whereas "lạc", which is not capitalized, means "peanut".