The Wagner Act, also known as the National Labor Relations Act of 1935, was a federal law that helped to protect American workers. This allows workers to organize into unions without penalty as well as giving them the power to negotiate wages/benefits with their employer (also known as collective bargaining).
The reason why this is often referred to as the "Magna Carta of labor" is due to the fact that both documents deal with gaining rights from a group of individuals within a society. In the Magna Carta, King John of England agrees to provide certain protections for a group of barons. These barons wanted protection from actions like illegal imprisonment. In this sense, you can see that the Wagner Act is similar, as workers wanted protection against being penalized for joining a union.
Essentially both documents are focused on giving more rights to a specific group of people who have been mistreated in the past.