Does this replace the cash register I already have?

For most small restaurants, yes. The POS takes the order, prices it with the same rules your ordering page uses, takes cash or card, opens your drawer, prints the receipt and counts the day at closing. What it does not do is take the money itself. Your card machine still does that, and cash is still cash. Think of it as the restaurant management software around the register rather than a box that holds money. The docs tab POS and the counter has the full picture.