What is the and $0xf, %reg pattern used for?

It masks off everything but the low 4 bits, giving a value 0-15 — equivalent to value % 16 or extracting a single hex digit.

* AND with $0xf clears all but the low 4 bits — an isolate-low-nibble mask equivalent to value % 16. *

movzbl (%rax), %edx
and    $0xf, %edx

This reads a byte (like a character) and masks it to get a value 0-15.

Common uses:

• Lookup table index: Extract low bits of a character as an array index
• Modulo by power of 2: and $(N-1), %reg = %reg % N (when N is power of 2)
• Bit field extraction: Isolate specific bits

Common masks:

Go deeper:

• Wikipedia: Bitwise operation — AND as a bit mask and modulo by a power of two
• Intel AND reference — bit-by-bit semantics and flag effects