Just bought the AC1000 (R6080) which states works with all N150, N300, N600 and AC devices

> Just bought the AC1000 (R6080) [...]

   That's "R6080 (AC1000)".  "AC1000" is a speed, not a model number.
Ever helpful, Netgear product packaging emphasizes speeds like "ACxxxx",
but that's not the model identifier.  Look at the product label.

> [...] which states works with all N150, N300, N600 and AC devices what
> does it mean by this?

   It means that your newer, faster router will still work with all your
older, slower client devices.

> I currently have a N600 which I replaced with AC1000. [...]

   Regarding "N600", see "a speed, not a model number", above.

> [...] Does this mean having both of them on as a mesh?

   No.  If you want to use your old, unknown-model ("N600") router to
extend the wireless network coverage of your new R6080, then you might
be able to employ it as a wireless access point.  Assuming that your
old, unknown-model ("N600") router is a Netgear product, visit
http://netgear.com/support , put in your (actual) model number, and look
for Documentation.  Get the User Manual.  Look for "access point".