When minimum wage rates per hour are set by law, it can raise the wages of those already in jobs and who manage to stay in those jobs. It has a negative effect on those who lose their jobs because firms no longer find them worth employing at the new rates. It has negative effects, too, on those trying to enter the labour market who do not yet have enough skills to be worth the minimum wage to potential employers.
Firms employ people because they are worth more to the firm than the wages they cost it. For low-skilled people their value to the firm might be quite low. Very often it is by starting on low wages and acquiring on-the-job skills that people move up the employment ladder. Someone who has worked has learned the importance of good time-keeping and following instructions. They have learned how the firm likes to do things, and are more valuable than an unknown potential employee. If the minimum wage is set at a level above that of their value to the firm, they find it difficult to secure those starter jobs.
In many countries those with low skills tend to be young people and sometimes those from ethnic minorities, especially if they have not had an adequate education. When minimum wage rates are increased, there often tends to be increased unemployment among these categories.
When minimum wages were introduced in the UK, the level was initially set sufficiently low that it had a minimal impact on employment. Subsequent increases are believed to have increased its impact, leading some economists to suggest that a better way of raising the take-home pay of low earners is to stop taking tax off them. Raising thresholds for income tax and National Insurance increases their wage without it costing employers money and pricing their services out of the market.