🧰 Waterproofing
Sealing the electronics and protecting the metalwork so a model can be run in wet conditions.
Sealed receiver boxes, waterproof servos and coated ESCs handle spray and puddles, but waterproof never means submerge-and-forget — motors, bearings and steel hardware still corrode afterwards. Salt water is far more damaging than fresh; rinse immediately.
What it actually does
Waterproof and water-resistant are not the same claim. Most "waterproof" RC electronics are sealed against splashes, wet grass and puddles — not against being submerged, and almost never against salt water. Knowing which you have decides where you can run.
In practice
Servos and ESCs are commonly sealed with O-rings and potting. Receivers are the usual weak point and are often protected with a sealed box or a balloon. Motors are generally not sealed at all: brushless motors survive getting wet but their bearings will rust if they are not dried and oiled afterwards.
Worth knowing
Salt water is in a different category — rinse everything with fresh water immediately afterwards, because salt keeps corroding long after things look dry.