what does 'double oughts' mean?

"Aught" and "ought" (the latter in its noun sense) strictly speaking mean "all" or "anything", and are not names for the number 0. Nevertheless, they are sometimes used as such in American English, for example, "aught" as a placeholder for zero in the pronunciation of the calendar year numbers.

As Wiktionary states -

aught (plural aughts)

1. whit, the smallest part, iota.

2. (archaic) zero

3. The digit zero as the decade in years. For example, aught-nine for 1909 or 2009.

The use of "aught" and "ought" to mean "zero" is very much proscribed as the word "aught" actually means the opposite of "naught": "anything". This may be due to misanalysis, or may simply be the result of unknowing speakers confusing the meanings of "aught" and "naught" due to similar-sounding phonemes.