Singlespeed bikes are bikes that are set up with one gear only. You can choose the size of gear upon building the bike. Or request a certain size to be put on the bike. This means there are no cables, no levers no derailleurs on the bike.
A single speed bike can be considered easier to ride as there is no shifting involved when riding the bike. People also choose to ride singlespeed bikes as there is little maintenance and fewer things to break, get worn out or service. They made popular choices for commuting along flat routes. Singlespeed bike setup can be hard work if you live in an undulating area as you cannot select different gears.