bibtex-autokey-titleword-ignore is a variable defined in `bibtex.el'.
Its value is

("A" "An" "On" "The" "Eine?" "Der" "Die" "Das" "[^[:upper:]].*" ".*[^[:upper:][:lower:]0-9].*")

Determines words from the title that are not to be used in the key.
Each item of the list is a regexp. If a word of the title matches a
regexp from that list, it is not included in the title part of the key.
Case is significant. See `bibtex-generate-autokey' for details.

You can customize this variable.