Objective significance doesn't exist, subjective significance is only significant to to the subjects in question, and everything else in this world that don't consider it significant will continue to move on as if nothing happened, whether or not said subjectively significant event took place or not.

Better question, why worry about significance at all? In the end, nothing we do will be significant to anyone, including the people who find it significant now.

If he or she does something that will change something for the generations to come.

lots of things. it could be a selfless act, a random act of kindness, anything that could help somebody or something or make them feel good. most things that seem insignificant to one could actually be extremely important to another person.

you are not who you think you are ... you are not who others think you are ... you are who you think others think you are ..... no seriously, loving yourself starts with your understanding the fact that God loves you, regardless of what you look like or what you ever accomplish in this world .... period

Reaching goals.

If they make even the tiniest impact on someone or something, their life was significant.

I think it is the impact that they leave on the world. How they are remembered, and how well they are remembered, and why they are remembered. This is the only real afterlife. The recreated image of people in the minds of our children and our childrens children.