What does this ARDLIB warning mean?

***ARDLIB warning: Filename some_file.fits does not specify an extension. 
Assuming the first "interesting" extension.

The warning is alerting the user that ARDLIB expected an input filename to have the desired block (aka extension) specified and it was not. For instance, "some_file.fits" was input instead of "some_file.fits[EVENTS]" or "some_file.fits[2]" (providing that EVENTS is the second block). If the block is not specified, ARDLIB uses the first "interesting" one; this is usually the EVENTS block in the case of an event file, the ASPHIST block for an aspect histogram, and EVENTS_IMAGE for a WMAP.

In most cases, e.g. the scripts mentioned above, this warning may safely be ignored. If you're not sure what the first block is, use "dmlist some_file.fits blocks" to list the blocks in the file. If a block other than the first interesting one is desired, use the syntax shown in the previous paragraph to specify it.