In an article on WDW News Today, the resort started testing with online reservations for the table service restaurants earlier this week in Disneyland Park, Walt Disney Studios Park, Disney Village and the hotels. Before this online possibility, the only options for making reservations were making a telephone call or reserving when you were already at the resort.
It looks like the new option is in a test period, because only the Dutch and Belgian version of the website seem to work fine, while the option for other countries does not always appear or work. Reserving a table is possible for a date one-and-a-half to two months from the moment you make your reservation, as when you choose to make a phone call this period is two months.
When it works, the option is very helpful for guests that want to maintain an overview of all their planned activities and want to make a reservation without the pressure of someone listening on the other end of the line.