The return on your investments can include both the growth in the value of your asset, such as the price of any shares you own increasing or if the market value of your investment property goes up, as well as from the income your assets might generate, such as rent or dividends. Different types of assets and different combinations of these assets will provide different levels of certainty about the level of return you might expect from your investments over a nominated time period. For example, if you invest in a term deposit, you can be certain what the interest rate will be that you earn on that term deposit over the time period you have nominated.
By contrast, the share market changes frequently. You may have an expectation of a rate of return when you purchase shares but the actual return across any period of time can be quite different, depending on whether the value of those shares increases or falls over the time period that you own them. In addition, not all shares will generate income through regular dividend payments, and for those that do - there is no guarantee as to what that payment will be.
There is a risk & return trade off which means that assets with a higher level of uncertainty have the potential for higher returns over a longer period of time than those with greater certainty of returns. Investments such as shares may offer higher returns over the longer term, but there is a greater inherent risk. In contrast, cash and fixed interest investments are considered to be less risky, but offer lower returns.