Refresh is he number of Fate Points your character has after a milestone. Generally, characters with high refresh are able to do more extravagant things while characters with low refresh are more gritty.
During adventuring your character will spend fate points. In order to gain them back you have to either have your aspects compelled or go through a milestone. milestones usually occur in between playing sessions, however the GM has full control. During a particularly difficult or long session the GM may allow a refresh part way through or if the party leaves in a gritty spot the GM may say there is no milestone.
When a milestone happens your fate points are set to your reset value. If you had more fate points than your reset value, they roll over.
During character creation or a milestone you can lower your reset value to gain stunts or you can sell lose stunts for higher refresh. Your refresh value can also be modified due to effective character level or consequences.