Someone who isn't weighed down by the troubles of everyday life, is always themselves regardless of the situation and lives life to the full. Not restricted by other people's opinions. Note: free-spirited should be used in a positive context.
A girl who still smiles when things aren't going too well, taking a positive outlook on life and trying to make the most of it - her spirit isnt damped by little things in life, she's free spirited.
Some girl that doesnt care what society thinks, is scrounging benefits, harassing old ladies and getting drunk in the park... she is NOT free-spirited, she has problems.