Conditional Formatting allows you to set a condition for a field, and then specify formatting based on that condition. For example, you could tell Access to format a field's text with red coloring if the field value is less than 0, or black if it's greater than or equal to 0.
You need to select a field that's bound to an underlying value or text. For example, you can't use conditional formatting on a label that contains text that has simply been typed in.