The positive aspect of labor unions is that they give workers the ability to pool their power and thereby exert pressure on employers in ways that individual workers cannot do. Unions make it harder for employers to act in ways that unduly exploit workers through, for example, low wages or poor working conditions.
The negative aspect of labor unions is that they tend to drive up the cost of labor past what it "should" be, according to market forces. Labor unions, once in place, tend to do things like setting up work rules that make workplaces less efficient than they could be. By doing this type of thing, labor unions drive up costs for businesses and, thereby, prices for consumers.