Video Editors are software that is used to create (and more importantly author) videos. Practically all Untergangers used a video editor to add (among others) parodied subtitles. Although a subtitling software can also be used, no Unterganger is known to make parodies this way (Subtitlecomedy did made use of Subtitle Workshop for some of his early parodies, but it was later replaced with Sony Vegas for his FX parodies).
Video editors have various degrees of ease and features, from low-tier Windows Movie Maker to god-tier Sony Vegas and even industry-standard tools such as Final Cut or After Effects. By far, the most famous video editor used for Unterganging is Windows Movie Maker, because they are shipped with most Windows versions.