300Mbps wireless adapter, often a USB dongle, allows a device (like a laptop or desktop) to connect to a wireless network at speeds up to 300 megabits per second. These adapters typically utilize the 802.11n wireless standard and are backward compatible with older standards like 802.11b/g. They are useful for upgrading or adding wireless capabilities to devices, and some even offer features like one-touch WPS setup for secure connections.