Please refer to the notes in EPRINTS_PATH/archives/ARCHIVEID/cfg/cfg.d/recaptcha.pl.
If this files does not exist, copy EPRINTS_PATH/lib/defaultcfg/cfg.d/recaptcha.pl.example to the path above and edit it.
This field uses the Google "reCAPTCHA" service (https://www.google.com/recaptcha/intro/) and renders a Captcha (a test that humans can easily pass, but robots shouldn't be able to).
Note: This MetaField was updated in October 2017 to reCAPTCHA v2. The previous version of reCAPTCHA will cease to work in March 2018.
As for Id fields.
validate:recaptcha_mismatch - Phrase to display if reCAPTCHA submission errored in some way.