That “indefinite” is linguistic terminology at a higher level of abstraction from the real world (there is nothing indefinite about the action itself — just the grammatical aspect is not specified). It just means that the simple verb is not marked for either perfect aspect or continuous aspect. Some people prefer that term because “simple” is not an aspect. The aspect of the simple verb is more like “other”, “unmarked”, or “indefinite” (undefined). To be honest, though, I feel like the term “simple” is more useful for most ESL students.