Open ticket means that the return date is not fixed. The date of the first flight is set (although typically this can be changed for a fee). Assuming that it is an unrestricted ticket that means that you can get a return flight on any date usually up to a year after your outbound flight.
You will need to call the airline to book a specific date once you know when you want to use the ticket. This will, of course, be subject to availability. You can not use it for a flight that is already fully booked. Assuming the flight isn't full, you could do this at the airport at the last minute.
To buy an open ticket you will usually have to go through the airline or a travel agent (i.e. not a website).
Restrictions vary with price. All open tickets must be used within a specific timeframe (at best 1 year). They are often only available for business class etc. Some have a fixed return date that can be changed for a nominal (or not so nominal) fee (useful if you have a 'I wont be longer than this' date but would like the option to return earlier). Ask before you purchase the ticket.
I should add that as these tickets are mostly used by business travelers, they are priced accordingly. They are rarely cheap.