It declares a variable named r10 and instructs the compiler to store it in the r10 CPU register. It's a GCC extension; the farthest you can get in standards-compliant C is

    register long r10 = a3;

but the register keyword is advisory only (the compiler is free to ignore it) and you cannot specify the exact register you want to be used.

Reference: https://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html