What is Capital Gains Tax and how do I know if I have to pay that Tax and how much I will have to pay on that Tax?

What is Capital Gains Tax and how do I know if I have to pay that Tax and how much I will have to pay on that Tax?

 

I am a renter in one state where I work and where I earn my W-2 income.

I also own a rental home, in another (different) state. 

 

I have owned that home in that other state for 7 years, where I lived 7 years ago (I lived in it for six months and then moved out of state for a new job.)

Ever since I have moved out of the home I own, I have had a renters living in my rental property (on and off) for rouughly the last 6 years.

 

Thank you so much for allowing me to ask this question.