Free-to-Play typically means that you can download, create an account for, and actively play a massive online game without having to spend any real world money. This means there are also no time-limits, or "Demo" periods, before having to pay. These types of massive online games typically have optional "Cash Shops" to cover continuing development and maintenance costs.
In Sword of the New World, leveling is technically free, requiring Cash Shop items to advance to the Veteran and Elite levels. These items can be traded amongst players using in-game currency, allowing some to avoid opening their wallets. One argument is that, since these items must initially be purchased with real world money, Sword of the New World is not actually Free-to-Play.