A stellarator reactor uses magnetic fields in a ring or donut type shape that twists in to order to confine and compress and thus sufficiently heat plasma to create fusion. The word “stellar” by definition “relates to the stars” where fusion takes place naturally.
The stellarator was invented at Princeton University by Lyman Spitzer in 1951. Early machines suffered from plasma leaks and stellarators were soon abandoned in favor of USSR developed tokamak reactors in the late 1960s. Compared to tokamaks, stellarators are harder to build but can produce a more stable plasma which is key to creating fusion.
Since the 1990s new methods for making and controlling high powered magnets has renewed interest in stellarators. Today there are several well known research stellarators. This includes the Wendelstein 7-X, the Large Helical Device and the Helically Symmetric Experiment.
Here is a great article that covers the basics of a stellarator.