What does "I'll drop you a line" mean?
Here's an extract from The catcher in the rye by J. D. Salinger;

Then we shook hands. And all that crap. It made me feel sad as
hell, though. "I'll drop you a line, sir. Take care of your grippe, now."
"Good-by, boy." After I shut the door and started back to the living room,
he yelled something at me, but I couldn't exactly hear him. I'm pretty sure
he yelled "Good luck!" at me, I hope to hell not.

Question

What does "I'll drop you a line" mean?
Here's an extract from The catcher in the rye by J. D. Salinger;

Then we shook hands. And all that crap. It  made me feel sad  as
hell, though. "I'll drop  you a line, sir.  Take care  of your grippe, now."
"Good-by, boy." After  I  shut the door and started back to the living room,
he yelled something at me, but I couldn't exactly hear him.  I'm pretty sure
he  yelled "Good luck!" at me,  I hope  to  hell not.

Answer

Hello Saeed. How are you?

"I'll drop you a line" =  I'll mail you a letter.

Did you get to the  page yet about Edgar Marsala?  My favorite part of the book.
I will never forget that page.

.