If you invest $100 in an index fund you should expect the traditional 7% annual return. So after one year, you have $107. You made $7 that year. If you leave it alone, you’ll get another 7% the following year. $107 x 7% = $7.49, bringing your value to $114.49, and so on and so forth.
On average, you should expect your initial investment in an index fund to double approximately every 10.3 years. This is why it’s important to invest early. If you invest $1 at age 25, with a retirement age goal of 65, that dollar will double, then double again, then double again, and double again to be worth $16. So every $1 you invest at 25 yo gets you $16 for retirement.