What is an enduro bike?

The name came about because those bikes were designed for endurance use, They needed to be very dependable, get decent fuel mileage, and have some speed. As important as speed is for a race it was not on the top of the list for endurance bikes, The top spot was given to dependability.
It will be a 'race' bike with lights and other items that are required for street use, Generally in the Street and trail category.
Most 'Enduro' bikes are in the 100CC to 250CC size range. and generally have a top speed of around 60 to 90 miles per hour.

An Enduro motorcycle is a motorcycle specially made for the Enduro sport, with the long travel and medium-hard suspension of a motocross bike conjoined with features such as a headlight and quiet muffler to make the bike street-legal for parts of the track. The engine of an enduro bike is usually a single cylinder 2-stroke between 125cc and 360cc[1], or 4-stroke between 250 and 650cc.

There can be an advantage to having an engine size smaller than 650cc in some Enduro events as it is nearly always lighter, which means it has easier handling for getting around trees, obstacles, etc. However, in some Enduro events, the larger bike sizes allow them to get up hills without going down a gear or even two.