The original use of the word stellar was in relation to stars or astral bodies. It was a commonly used word to describe bodies of stars for instance ‘stellar regions’ would be full of stars. In more common language, it is now used to describe something that is worthy of star status. A stellar experience would be one that stands out in your mind as being as bright as a star. A stellar performance by a actress would be the same, something worthy of a label that is equal with the brightness of a star.