A wireless repeater is a network device for propagating wireless network signals beyond the typical boundary of a network. Wireless repeaters are generally passive devices and can only bridge connections between the resident network and the host pursuing access to the network. This is called increasing the the network's stretchability.
A wireless repeater may also be known as an expander.
Techopedia explains Wireless Repeater
Wireless networks are powered by access points or wireless routers, which propagate wireless signals in an organization ora residence. Upon authentication, hosts such as computers or iPhones are provided Internet connectivity, and may be moved within the premises without losing connectivity. However, with the advent of hand-held gadgets, laptops and other Wi-Fi-supported devices, users sometimes reach out-of-access-point limits and lose the wireless signal. Wireless repeaters are used to cater to these remote users, and are implemented at access point boundaries.