Repeaters:

The term repeater comes from the early days of long distance communication. The term describes the situation when a person on one hill would repeat the signal that was just received from the person on the previous hill. The process would repeat until the message arrived at its destination. Telegraph, telephone, microwave, and optical communications use repeaters to strengthen signals sent over long distances.
A repeater receives a signal, regenerates it, and passes it on. It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media.

One downside of repeaters, though, is that they reduce throughput on the LAN. A repeater must receive and retransmit each frame on the same RF channel, which effectively doubles the number of frames that are sent. This problem compounds when using multiple repeaters because each repeater will duplicate the number of frames sent. Thus, be sure to plan the use of repeaters sparingly. The Four Repeater Rule for 10-Mbps Ethernet should be used as a standard when extending LAN segments. This rule states that no more than four repeaters can be used between hosts on a LAN. This rule is used to limit loss added to frame travel by each repeater.
As a result, repeaters are an effective solution to overcome signal impairments such as RF attenuation. For example, repeaters provide connectivity to remote areas that normally would not have wireless network access. You may have one access point in a home or small office that doesn't quite cover the entire area where users need connectivity, such as a basement. The placement of a repeater between the covered and uncovered areas, however, will provide connectivity throughout the entire space. The repeater fills holes in coverage, enabling seamless roaming.