Summits on the Air (SOTA) is an award program in which operators climb listed mountain summits, set up a small station on top and make contacts. The climber is the activator; the stations who answer, often from home, are chasers. The radio is the lightest part of the load: the exercise, navigation and weather are the main event.
Why a summit
Illustrative. The altitude gives line-of-sight reach; the weather and daylight limit your time.
Height does two things. For VHF and UHF it extends line of sight over lowland, so even a handheld with a small antenna can reach a long way (see Line of sight and the radio horizon). For HF, a clear summit has little around it to block or absorb the signal, and a low take-off angle helps with distance (see Ground effects and height). The program lists each summit by a reference, and points typically depend on the summit's height. Details change: check the current rules.
The station
Radio. A handheld for FM on VHF, or a lightweight HF transceiver. Low power (QRP) is common; see QRP and portable radios.