Before the flood people lived longer. Adam died at age 930 years (Gen 5:5), Noah at 950 years (Gen 9:29) and Methuselah (Noah’s grandfather) was the oldest of them all, dying when he was 969 years old (Gen 5:27). There is no reason to think that these “years” were any different from our years.
After the flood, the lifespans seemed to have rapidly decreased, although for a few centuries people were still living well into their second century.
There have been many speculative explanations for this change, but we are simply not told why God chose to reduce human lifespans.