suffix
every ...; at intervals of ...; ... apart
Attaches to a noun indicating a distance, time period, or number of objects to express regular intervals. Often translates as 'every ...' or 'every other ...' depending on context. For example, 1メートルおき means 'at intervals of one meter' or 'every meter'.
この道には10メートルおきに木が植えてある。
Trees are planted every 10 meters along this road.
この薬は6時間おきに飲んでください。
Please take this medicine every six hours.
He goes to the gym every other day.
ごと attaches to nouns to mean 'every' as well, but emphasizes each individual unit without exception (e.g., 日ごと = every day). おき focuses on the interval between occurrences.
Derived from the verb 置く (おく, 'to put, place'), suggesting the idea of placing intervals. The suffix usage developed from the concept of leaving a space or time between things.