How does a sextant help determine latitude and longitude?

A sextant is a navigation instrument used to measure the angle between two visible objects. An example of the two objects might be the Sun and the horizon, or the Moon and a particular star at night.

Using the angle and the time it was measured, it is then possible to calculate a position on a map or chart (latitude and longitude).

Time needs to be measured precisely, so ships would carry a chronometer and this also explains why navies and seafarers were so concerned about timekeeping.

Sextants can be used on moving vessels, because they measure relative angles and so both the sun and the horizon would move to the same extent.

https://wiki2.org/en/Sextant+Newton