On a transmission line, the signal suffers from distorsion, and becomes weaker as the distance is between the two active elements becomes longer. Two local area network nodes are usually no further than a few hundred meters apart; this is why additional equipment is needed to place nodes beyond that distance.
A repeater is a simple device for refreshing a signal between two network nodes, in order to extend the range of a network. The repeater works only on the physical layer (layer 1 of the OSI model), meaning that it only acts on the binary information travelling on the transmission line and cannot interpret the packets.
What's more, a repeater can be used as an interface between physical media of two different types, meaning that it can, for example, link a length of twisted-pair wire to a fibre-optic line.