[action] over a period of time. Means that during the time you do [action]. example : I was teaching over a period of 10 years.

As the previous answers indicate, ''over" preceding a time refernce, has the same meaning as ''during." A common English idiom is "over the years.'' 示例: He's changed a lot over the years. (= during many years)

during a continuous 10 year period - often they will say "Over a 10 year period from 1976 . . . . . . " or "We got to know each other over a period of 10 years."