So most of the ACS samples from 2000 to 2015 are said to be “1-in-100 national random sample of the population”. What does this exactly mean/imply about the sample?

# what does "1-in-100 national random sample of the population" specifically mean? what is the interpretation?

This shows how large the sample is relative to the total size of the population. For most recent US census years, a 1-in-100 random sample of the population means that the sample is approximately 1% of the size of the total population. Additionally, although this varies based on demographics, in a 1-in-100 random sample each person - on average - represents 100 people in the total sample. Practically, what this means is that you’ll need to use the sample weights (PERWT or HHWT) if you wish to calculate nationally representative statistics.