Macro is an assembler directive, is the sintax to do a macroinstruction,read metalink documentation.

For example if you use a lot of times one operation that is:
mov a,addr
add a,#3
mov addr,a

you can make a macro, named for example "my_op" and do:

my_op: macro ;here list of parameters
mov a,addr
add a,#3
mov addr,a
endm

The next time you must use thouse instructions you can use the "opcode" my_op instead all instructions.
The assembler copyes all the instructions for you.