The autocomplete shows you a list of elements that can be used to build you expression. The list shows only allowed elements and dynamically changes based on what you have already entered and what is more likely to be entered next.
To use autocomplete, start entering the expression, and the drop-down list with suggested completions will appear at the bottom of the field. Depending on what the next element of the expression must or can be, suggestions consist of either properties or operators. Click the value to insert it into your expression.