The good news is that Room Service is still free (mostly) on Holland America Line ships (they're charging for it on most mainstream cruise lines). Free is good. We like free.
The less-good news is that the Room Service offerings are relatively limited (as they tend to be on all but the super-luxury cruise lines).
Also less good is that a few items DO carry a surcharge (e.g., food from the Dive In or Pinnacle Grill menus).
Room Service is available 24-hours a day on Holland America ships by pressing 92 on your cabin phone. With the exception of a few specific extra-cost items, Room Service remains without any additional charge. (I should add, however, that it's customary to tip $1 to $2 per person being served to your server.)

As with every cruise line below the super-premium level, the Room Service offerings are relatively limited, certainly in comparison to what you can get in The Dining Room or the Lido Market. But the bottom line is, no matter what the time of day, you'll be able to get something to eat. There is no opportunity to die of starvation on a cruise ship!
Breakfast is available from Room Service between 6 AM and 11 AM.
At breakfast, you have two options available. If you know what time you'll want it served (and that time is no later than 10 AM), you can put out the doorknob hanger your room steward will leave for you when he makes up your room in the evening. What you've ordered will be delivered at the time you've requested (give or take 15 minutes). My experience is that it's served on the dot at your requested time.
If you aren't sure what time you'll get up, or if it's between 10 AM and 11 AM, or if you don't know what you'll want to eat in the morning, you can simply press 92 when you wake up (as long as it's before 11 AM) and order from the Room Service Breakfast menu.

There's no difference in what's available. Everything you can order from the Room Service Breakfast menu is available on the door hanger card (and vice versa). The biggest difference is that it's going to take at least 30 minutes from the time you wake up and order to get your meal.
By the way, you can make special requests on the door card, and they'll be accommodated if it's possible to do so. For example, I like to have a raisin bun (HAL makes incredible raisin buns) mornings with my tea at 6 AM, so I just write it in on my door card, and it's always there when the steward delivers my tea. So if you'd like two croissants or some extra bacon, write it on the card.
Between 11 AM and 11 PM, you can order from the All Day Room Service menu. You can get such things as a quesadilla, soup, chili, a salad, pasta, roast chicken, a sandwich or something sweet.

During the hours The Dining Room is serving dinner (5:30 PM to 9:30 PM), you can also order anything off The Dining Room menu (which you can see in The Navigator App or on your cabin television).

Also available during the dinner period (5:30 PM to 9:30 PM) is a special FOR KIDS menu with several kid-friendly items like mac and cheese, a hot dog, chicken nuggets, and a peanut butter and jelly sandwich.
After 11 PM (until 6 AM), there's a the LATER menu which is a subset of the ALL DAY menu (making the limited offerings even more limited).
There is also a page of the Room Service menu with offerings from Dive In (hamburger, cheese sandwich, chicken sandwich, and hot dog) and the Pinnacle Grill (New York Steak, Filet Mignon Steak, and Lobster Tail). The Pinnacle Grill items are available from 5 PM to 9:30 PM, but the Dive In burgers and other items are available 24-hours a day. The catch is that they all come with an extra cost ($5 for the Dive In items, $15 for the steaks, and $20 for the Lobster)*.
As I said, you won't starve
* Restaurant surcharges are current as of the moment, but are subject to change.
I should also mention that there are a few extra perks if you're in a Neptune Suite or higher (or have Club Orange membership). First you can get a Mimosa or fresh-squeezed orange juice with your breakfast order without any extra charge. You can have Afternoon Tea served in your suite each day without charge. And you can have complementary hors d'hourves served in your cabin each evening before dinner.