It is the idea of what should be paid to a worker. The wages which are set by market drivers of demand and supply and are also justifies by requirements for the job like expected experience, educational qualifications and skills. It is the level of wages which is high enough to incentivise workers and low enough for employers to profitably offer the job.
It is difficult to actually calculate the exact value of the justified wage level as there are different methods and people may value different requirements for a job in a different manner. The difference in justified wage level and minimum wage level is dependent on the policy and priorities of the state, the level of unemployment and the state of economy.
For example, the level of justified wage for a car wash mechanic with 2 years of work experience may be $15 during normal times. But in recession, the wage level they get might drop close to the minimum wage.