Every time you run as it produces an output file, which is
your assembly language program translated into numbers. This file
is the object file. Its default name is a.out.
You can give it another name by using the -o option. Conventionally,
object file names end with .o. The default name is used for historical
reasons: older assemblers were capable of assembling self-contained programs
directly into a runnable program. (For some formats, this isn't currently
possible, but it can be done for the a.out format.)
The object file is meant for input to the linker ld. It contains
assembled program code, information to help ld integrate
the assembled program into a runnable file, and (optionally) symbolic
information for the debugger.