Refers to being outside, exposed to the sky, often in contrast to being indoors or under cover. Commonly used in compounds like 野天風呂 (open-air bath).
Eating outdoors feels great.
This inn has an open-air bath.
open-air bath
Compound of 野 (field, wild) and 天 (sky, heaven), literally 'field sky', referring to the open sky above a natural area.