From left to right (or top to bottom on a vertical toolbar), the buttons are:

Readable toolbar with numbered labels: 1 Play/Stop, 2 Hover menu, 3 Voice input, 4 Text appearance menu, 5 Predict menu, 6 Check menu, 7 More menu, 8 Settings, 9 Help.
Every button on the Readable toolbar (shown Large, in Colours). The toolbar can be horizontal or vertical — the buttons are the same.
Play / Stop
Reads your selected text. If nothing is selected, Readable starts from where you last clicked and reads to the bottom of the window. While Readable is speaking the button turns into a red Stop button. Shortcut: ⌥ Option+⌘ Command+P. See Play and Read Selection.
Hover ▾
Chooses the hover reading mode — Hover off, Word hover, Line hover, Sentence hover or Document hover — with a tick beside the current one. It also has Text extractor (OCR) and, once you’ve chosen a sound, Ambient sound to switch it on or off. See Hover Read Modes.
Voice input (microphone)
Starts and stops dictation into the app you’re typing in. See Voice Input.
Text appearance ▾
Font, size, spacing and colour for your document. Only shown when Microsoft Word or Apple Pages is in front. See Word and Pages Text Appearance.
Predict ▾
Switches Predict words and Autocorrect on or off. See Word Prediction and Autocorrect.
Check ▾
Look up, Thesaurus / synonyms, Show homophones and Clear homophones. See Word Tools.
More ▾
Reading ruler (when Sensory Ruler for Mac is installed), Save to audio file… and Spoken-text banner. See Reading Ruler, Save to Audio File and Spoken-Text Banner.
Settings
Opens Settings, arranged in tabs: Speech, Reading, Banner, Predict, Ambient, Toolbar and About. See Settings Reference.
Help (?)
Opens the Readable help.
Hover dropdown listing Hover off, Word hover (ticked), Line hover, Sentence hover, Document hover and Text extractor (OCR).
The Hover dropdown. The tick shows the current mode — Word hover by default. Ambient sound also appears here once you choose a sound in Settings.
Check dropdown listing Look up, Thesaurus / synonyms, Show homophones and Clear homophones.
The Check dropdown. Show and Clear homophones are available when Word or Pages is in front.
More dropdown listing Reading ruler, Save to audio file and Spoken-text banner.
The More dropdown. Reading ruler appears when Sensory Ruler for Mac is installed.