"To put the power of compounding on your side, you have to start early."
This is said of earning interest, and such. "Compound" here means the first year you get, say, 5% over 100%, nexy year 5% over the last 105%, etc. (in technical terms, you earn interest on the 'principal' and 'accrued' interest). So, your phrase is used for savings-plans and such.
sth = something.