indirect indexing looks like this:
lda (addr_lo),y

But a few assemblers use brackets instead of parenthesis.

What you'd do to use it, is put the address to your nametable data in 2 bytes of zeropage-RAM. We'll call them addr_lo and addr_hi. You'd do that (in most assemblers) like this:

lda #sta addr_lo
lda #>mapdata
sta addr_hi

That's the 'indirect' part. The indexing part uses the Y register, and is an index into the address that we have in addr_lo,addr_hi

A way to use it would be like this:

ldx #4 ; (load 256*4 bytes)
ldy #0
loop:
lda (addr_lo),y
sta $2007
iny
bne loop
inc addr_hi
dex
bne loop

See how that works? Indirect indexed mode is also useable with several other instructions, it can be really useful.