A hydrometer is a weighted, sealed glass float with a graduated scale printed inside its stem — dropped into a sample of wort or beer, it floats higher in denser (higher-gravity) liquid and lower in less dense liquid, and the reading is taken where the liquid surface crosses the printed scale.
The two things that trip up a first reading
A hydrometer needs a sample deep enough for it to float freely without touching the sides or bottom of the container, and it needs to be read at eye level at the meniscus (the slight curve where liquid meets the glass), not glanced at from above — both are easy to get wrong on a first attempt and both produce a genuinely inaccurate reading if missed. The other common error is not correcting for sample temperature; see gravity temperature correction for why a reading taken hot or cold without adjustment doesn't match what the same hydrometer would show at its calibration temperature.
Where a refractometer earns its keep instead
A hydrometer needs a reasonably large sample and a few minutes to settle before reading, which is why many brewers switch to a refractometer for quick checks during an active brew day, reserving the hydrometer for the OG and FG readings that matter most for the final ABV calculation.