"The secret to happiness is freedom... And the secret to freedom is courage." - Thucydides
Those ancient philosophers , historians , writers 
etc had some amazing quotes.
The thing about quotes is that most of the time they are vague, and the reason for this is that at any given moment in life, those quotes can be interpreted differently due to the circumstances and events happening in anyone's life at that moment. If you were to write about that quote every five years or even just once, I am sure the answers could be different. But let's focus on how I interpret it:
The secret to happiness is freedom. For me, if someone wants to truly be happy in their life, they must have freedom. Freedom means having no limitations or restrictions on the choices one wants to make regarding how they live their life. When that happens, it is more likely for people to experience a genuine sense of happiness and fulfillment.
The secret to freedom is courage.
Of course, for someone to attain freedom, they must have a lot of courage. As humans, we do not want others to be truly free, which is why we create so many rules and criticize those who try to live their life the way they want because it is seen as abnormal. So, if someone wants to have freedom, they must first have the courage to confront and overcome obstacles and challenges that might try to suppress that freedom. Courage is the inner strength and resilience required to stand up for one's beliefs and desires.
Of course what freedom is might differ from people to people. In a poor country freedom would come when the financial problems where solved.
In an oppressive society being able to choose how to live you life and make your own choices would mean freedom
And if you ask people with different nationalities , gender etc i believe the answers would be different.