Prelude uses a few words in its own way. Here is what each one means, in plain words.

Library: the page you see after you sign in. It lists all your music and is where you add new pieces.
Correct: the page that shows your own scan with what Prelude read drawn on top in colour. Here you fix anything it misread.
Reflow: your clean new copy of the part, redrawn from scratch. Here you make it easier to read (bigger notes, letter names, paper colour), play from it on screen, and print it. "Reflow" just means the music can flow onto the page at any size.

Reading (sometimes called detection): Prelude looking at your scan and working out the notes, rests, clefs and other symbols. "What Prelude read" or "detected" means what it found. It is good but not perfect, which is why there is a Correct page.
Mark: any symbol on the page: a note, rest, sharp, accent, bowing, dynamic and so on.
Tag / tagging: telling Prelude what a symbol is. When you place a symbol on the scan, or accept one of its guesses, you have "tagged" it.
Untagged ink: a purple dashed box around ink that nothing explains yet, such as a missed symbol, a pencil mark or a speck. You either tag it or tell Prelude it isn't music. See What are the purple dashed boxes ("untagged ink")?
Gold (yellow) marks: accents, bowings, dynamics and similar marks that Prelude read. See What are the gold (yellow) marks?
Mark wrong / ✕ Delete: telling Prelude something it read isn't really there.

Layer: one kind of mark (notes, stems, rests, clefs…). Each layer has a button and a colour, and you can hide or show it.
Status line: the short grey sentence under the buttons. It always says what just happened or what to do next. Read it when you're unsure.
Palette: the list of symbols on the left that you can add to the page.
Red pill: a small red label under a bar whose notes and rests don't add up to the time signature, so something in it is probably still wrong. A grey not checked pill means Prelude couldn't count that bar yet.
Voice: used only when two separate lines of music share one staff. Most players can ignore it.
Deep-dive zone: an orange box you draw around a spot you can't fix, with a note to say what's wrong.

Bar (also called a measure): the music between two barlines. Prelude says "bar".
Value: how long a note or rest lasts: whole, half, quarter, eighth, 16th, 32nd.
Note head: the round part of a note. Letter names go inside it.
Ledger lines: the short extra lines above or below the staff for very high or very low notes.
Accidental: a sharp (♯), flat (♭) or natural (♮) written in front of a note.
Key signature: the sharps or flats at the start of each line, just after the clef.
Rehearsal mark: the big letter or number (A, B, C…) your director uses to say where to start.
Insert: an extra page or slip with new or replacement bars, often handed out by a director. See How do I add an insert?

Engraved / re-engraved: printed as clean music notation, like a publisher would, instead of a photo of the old page.
Edit bar ▾: the button that appears on a bar when you click it. It opens the bar menu: everything you can change for that bar, in groups (Notes, Accidentals, Clef, Transpose, Measures, Copy & paste, Layout, Add a mark). Click a group to open it; ‹ Back returns to the list.
Transpose: reading a passage written for another instrument key (such as "Horn in E") in the key you play. See How do I read a passage written for another instrument key?
More ▾: the button at the top that opens more settings (note size slider, bars per line, fingering, portrait or landscape).
✎ Edit music: a switch that lets you change the notes themselves on your clean copy, for example to simplify a passage. It never changes your scan.
Read (Read mode): a clean screen with only the music, for playing. Tap the right half to turn the page.
Page break: the place where a new page starts. You can choose it yourself.
Undo / Reset: Undo takes back your last change. Reset (under More ▾) takes the whole Reflow part back to the plain engraving (your note size stays). See How do I undo a change or start over?

Alpha: an early test version. Prelude is in alpha. See What to expect while Prelude is in testing
Feedback tab: the dark red tab on the right edge of every page, for reporting problems or asking questions.
Queue: the line of pieces waiting to be read. Yours starts by itself when its turn comes.