You're right; that statement is too vague to understand what's involved.
If you want to park a phone number ("park" is defined as "I really don't plan to use this number, but I want to keep it active in case I need it again in the future"), then you'd port it to Callcentric and sign up for their pay-per-minute inbound calling option, at $1.95 per month. This would at least enable inbound calling, which could, for example, be sent to their free/included voicemail. If you wanted to occasionally call out on that number, or forward that number to some other number, then you could add the North America Basic plan.