Transcription is getting a text from a medium (which can be from a facsimile, a photo, an audio or video recording). This is at the heart of several Wikimedia projects, Wikisource obviously but also Commons and in the end, it can be found in one form or an other in all Wikimedia projects.
Transcribing is fundamental for a Gallery, Library, Archive or Museum, as it allows media contents to be more accessible, making these contents to be visible and discoverable. It has specific needs that requires special tools (OCR, Proofread Page extension, TimedText, templates like Inscription, and many others). Transcription is also done outside Wikimedia constellation, in several projects that can be an inspiration for us.