htdigest needs the next parameters:

Usage: htdigest [-c] passwordfile realm username

What's the realm for? What value must i use to create htdigest?

Anyway, realm denotes authentication domain for chalenge-response. Within the situation of digest authentication realm value takes part within the password hash, that why it's needed when storing password files.

See http://tools.ietf.org/html/rfc2617#section-3.2.1 for particulars.