What it does
Segol (sometimes written Segolta) is a strong disjunctive that appears in long verses. It marks a major sub-phrase boundary, usually in the first half of the verse before the Etnachta arrives. When you see Segol, the verse is going to be long.
Where it appears
Less common than Etnachta or Zakef Katan. Found mostly in long verses where the first half needs an additional break. Segol is led in by Zarka, which usually appears one or two words earlier.
How to remember it
Visually: three small dots arranged in a downward triangle above the letter. The shape matches the Hebrew vowel of the same name (segol nikud has the same three-dot pattern). Some teachers say the three dots represent the three "beats" of the longer pause.
Example from the Torah
Bamidbar 7:89
When Segol appears, it typically caps the first major clause of a long verse. The Zarka before it pulls into the Segol pause, much like Tipcha pulls into Etnachta.
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
Zarka leads into Segol as its servant. They appear together: Zarka on an earlier word, Segol on the word that ends the sub-phrase.