What is the significance of labor union?

My answer is labor union is significant because the major importance of a labor union is to organize the workers in order to make available a organization to negotiate a collective bargaining agreement with the management.

Is this adequate?

I suggest you add that labor unions give power to the workers that offsets management's power. Labor unions help the balance of power.

So then, a labor union is significant because they give power to the workers that offsets management's power and helps the balance of power?

Right.

Labor unions were started to help laborers whose bosses mistreated and underpaid them.

Yes, your answer provides a brief explanation of the significance of labor unions. However, I can provide a more detailed explanation if you'd like.