How would you describe the meaning to the phrase, "Stay in your lane, girl!"?

It means to focus on your life and accept all that comes with it. Moreover, it also means to not focus on what others are doing.

http://www.vibe.com/2013/06/vixen-inspiration-stay...

It means to carry on doing what you have been doing, just exacly the way you have been doing it.

And not to veer over into somebody else's personal affairs.

http://letskeepitauthentic.blogspot.in/2013/03/sta...

to mind one’s own business; to keep moving straight ahead without veering into others’ personal affairs.

https://boldandfab.wordpress.com/2014/02/19/stay-i...

it means do not go further or know your limits. depends on th context

It's like say; '' You're on the good path''. ( understood: continue like that).