A more thorough explanation:

United Steelworkers of America v. Weber (1979) is a U.S. Supreme Court case that dealt with affirmative action programs in the workplace. The Court ruled that Title VII of the Civil Rights Act of 1964, which prohibits racial discrimination in the workplace, does not prohibit voluntary affirmative action programs that seek to hire minorities who were historically underrepresented.

For example, in this case, the United Steelworkers of America (USWA) and Kaiser Aluminum and Chemical Corporation entered into a collective-bargaining agreement which provided that Kaiser would fill 50 percent of craftworker trainee positions by black workers until the proportion of black workers at Kaiser mirrored the local labor market pursuant to the Title VII affirmative action provision. Previously, black workers had been unable to enter Kaiser’s craftwork force because black workers lacked craftwork experience from being historically excluded from craft unions. A white worker, Weber, was not selected even though he had greater seniority than many of the black workers selected for trainee positions. Weber sued USWA and Kaiser for violating Title VII for discriminating against white workers.

The Supreme Court ruled that voluntary affirmative action hiring programs that seek to increase minority worker participation in industries which historically excluded minority workers do not violate Title VII. While Title VII prohibits racial discrimination regardless of which race the aggrieved person belongs to, the Court reasoned that the legislative history of Title VII showed a special focus for ensuring black workers can secure jobs in industries they were historically excluded from. Additionally, Congress specifically stated that Title VII does not require employers to adopt such affirmative action plans, so if they meant Title VII to prohibit the voluntary adoption of such plans, it would have explicitly legislated so.