What it does
Zakef Katan ("small upright") is a medium-strength disjunctive. It marks a meaningful pause within a clause, weaker than Etnachta but stronger than Pashta. After you hear Zakef Katan, you know the next short phrase is starting.
Where it appears
Very common: most verses have at least one Zakef Katan, often more. It usually sits at the end of a short phrase and is preceded by a servant trope (commonly Munach) one or two words earlier. The pattern Munach-Zakef Katan is one of the most-heard sequences in all of Torah reading.
How to remember it
Zakef means "upright" or "standing tall." The mark is two small vertical dots stacked above the letter, like two pillars standing at attention. Katan ("small") distinguishes it from its bigger cousin Zakef Gadol.
Example from the Torah
Shemos 3:4
Notice the Zakef Katan on each call of "Moshe Moshe" in this verse. The mark separates the doubled name from the rest of the sentence, giving each call its own beat.
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
Zakef Gadol has the same two-dot mark plus an additional vertical line beside it. Functionally similar but slightly stronger. Zakef Gadol replaces Zakef Katan when no servant trope precedes it.