New investor looking for answers, what do these things mean?

[Liam Marshall]

I am 17 years old and very new to real estate investing. As I listen to more podcasts, read more books, and watch more videos, I have more and more questions. The following are things I heard or read that I didn't understand and did not get a very clear or simple answer from google. 

What does it mean to under write a multifamily?

Cash out?

Cash out refinance?

Balloon Payment?

I understand the concept of buying properties with value add opportunities because appreciation now affects the property at a higher price and I understand that through this you also build equity because with a 30k value add instead of having 20k equity in a 100K property you have 50k equity in a 130k property. (I think but I could be wrong on both) But what does this actually mean? How can you use this to your advantage? Can I take out a HELOC and use 70% of my 50k to put a down payment on a new property? (just a hypothetical) Or does that not work because in the eyes of the bank do I still only have 20k in equity? A lot of people talk about how important equity can be but without a real understanding Its hard for me to see that importance.

Sorry for questions that probably seem elementary to most of you, just looking for some clarity.