What it does
Munach is a conjunctive trope that "serves" the disjunctives that come after it. Where Mercha tends to lead into Tipcha or Sof Pasuk, Munach is more versatile: it serves Etnachta, Zakef Katan, Revia, Pazer, and several other disjunctives. If a disjunctive is coming up and you need a servant to lead into it, the answer is often Munach.
Where it appears
Throughout the Torah, often multiple times per verse. The pattern Munach-Etnachta is one of the most recognizable in the entire system: a servant note that drives directly into the major mid-verse pause.
How to remember it
Munach means "laid down" or "at rest." The mark itself is a horizontal line laid flat beneath the letter, almost like the letter is resting on it. Think of it as the foundation that other trope marks build on top of.
Example from the Torah
Bereishis 1:1
Munach sits on בָּרָ֣א, leading directly into אֱלֹהִ֑ים, the word that carries Etnachta. "Munach-Etnachta" is the most common disjunctive pairing in the Torah. Once you hear it, you'll never miss it.
Hear the melody
A synthesized rendering of the melodic shape, not a vocal recording. For a baal koreh's voice on a full aliyah, PocketTorah is a great free resource.
Hand signal (simanim)
Often confused with
Mercha and Munach are both very common conjunctives that sit below the letter. The shortcut: Mercha is a diagonal stroke (it leans); Munach is a horizontal stroke (it lies flat).
Both Mahpach and Munach are below-the-letter conjunctives. Mahpach is an angled bracket; Munach is a straight horizontal line.