Beginner Level 2

Hello all

 

Hope you're all well.

 

I'm now beginning to get into understand my photo editor pro on my phone and I came across the tone curve. Now I understand what it means but what I'd like to know is what do the three dots (black, grey and light grey) at the bottom mean and what do the red, green, blue dots on the left of the graph mean? I'd also like to know what the three lines in the middle of the graph mean. 

 

Would anyone be so kind to explain this to me so that I know what I am doing with my photos when I edit them! 

 

Thanking you kindly :)