Modular helmets are helmets that cover a biker's whole head (called full-face helmets), but which have a front section that can be flipped up, temporarily transforming them into open-face helmets. Being able to flip up the front of the helmet makes them easier to adjust and more comfortable to wear than traditional full-face helmets. Additionally, it does not reduce the safety of the helmets because while riding they are flipped back down to become like full-face helmets.
Since modular helmets act like full-face helmets when they are worn, they are indeed safe to wear so long as they are worn properly. When riding with a modular helmet on, do not flip up the front section of the helmet when riding, and do not ride with it flipped up. It should only be flipped up when the bike is at a complete stop and should be put down before you begin riding.
The purpose of the modularity of the helmet is to provide riders with more comfort and adjustability options while still maintaining the safety features of a full-face helmet.