Condition - Formula is

Applies the selected style to the cell when the formula expression in the text box in the right is not zero.

The formula is expressed similar to a test condition evaluating to TRUE or FALSE.

For example, if C29 has a conditional formatting which formula is B1>=(DATE(2015;5;4)+TIME(18;0;0)) - a logical expression that evaluates true or false - the style applied to C29 depends on B1 contents.