The autocomplete shows you a list of elements that can be used to build your 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 typing in the expression until you see suggested completions in a drop-down at the bottom of the field. Depending on the next element of the expression, suggestions can consist of properties or operators. Click the value to insert it into your expression.