NTP stands for Network Time Protocol. NTP server is a device/service that provides accurate time synchronization to other devices on the network. These servers can be located both on the public internet and locally on private networks. The main purpose of an NTP server is to ensure that all devices on the network have a consistent and accurate time.
In context of NETIO products:
NETIO devices are using NTP server to synchronize its internal time. This is important because time is necessary for proper scheduling of output states and for evaluating conditions related to time. NETIO devices are designed to be networked for remote control, which means that they assume an NTP server is available in the network and can synchronize their time whenever needed. However, because of this assumption, most Netio devices do not have a time battery backup and relies on NTP server. Only PowerPDU 4C has battery backuped time.