A chart is a set of notes that go to the audio file presented along with it. The term “Chart” has a few other words close in meaning, such as “File”, “Map”, or “Simfile”. While they are sometimes used interchangeably, their meanings are a little different - by definition, “File” and “Simfile” refer to a group of charts for one song. “Map” implies it's an osu!mania chart.
The most recommended program is ArrowVortex, a free editor with a graphical interface. One may also choose DDReamStudio, an older alternative. The ingame editor, available in other forks of StepMania 5, was deleted from Etterna to encourage users to chart with external software, which is more advanced and offers more possibilities.