Optical Network Terminal is a box where the fiber optic line runs into, and terminates within the premise either as a coaxial connection, or RJ45. When terminating as a coaxial connection, a FiOS modem will be required. Most ISPs will provide router features with their modems therefore causing issues if you wish to use your own router. For connections that terminates as an RJ45 connection, you would simply plug your router into the RJ45 jack, and you will have internet.
Keep in mind, if you wish to have FiOS Telephone, or want technical support, you would have to use the ISP’s hardware. Your ISP will likely only provide technical support for their routers, so you would have to troubleshoot and make sure it’s not your router that is causing the problem.