Sketch uses the macOS coordinates system, which is measured in points. So, when you work in Sketch, everything is measured in points — this includes text layers.
A single point in Sketch translates to a single physical pixel on an @1x display. When you export layers, you can change the output scale to whatever you want. So if you export using an @2x scale — for high-resolution “retina” displays — a single point will be equal to two pixels. If you export using an @3x scale, a single point will equal three pixels, and so on.
If you copy CSS attributes from the the Mac app or from the Inspector panel in the web app, these will be measured in px. CSS px measurements are not the same as physical pixels (as this article from W3C explains) but you can think of CSS pxmeasurements as the equivalent to points in Sketch.
Points in Sketch are not the same as points (often abbreviated to pt) that are used as a unit of measurement for typography. The pt unit of measurement is a fixed size (1/72th of an inch), whereas the size of a physical pixel is based on the resolution of a display.
