Basically, a rule is called from a single field or line layout in a form (look under subobjects on pe51 screen) . What a rule does is compare a field from a table to a value you specify in column 'value 1'. If the field is true, then it populates the field on the form with the value specified in columns 'table' and 'new value'.
For example, rule #2 is comparing FPBEG in table XRT to 'EVP-BEGDA' (which is pay period begin date) and if they are equal, the field will be left blank (because nothing specified in 'value' column) or if they're not equal, then the field will populated with constant 'P'. This is why rule #2 has 2 parts.
You'll see that there are no values in column offset and comp.length and these fields are only used if you want to compare certain chars in the string. If you want to use a constant in 'value 1', you use a ' char before the value which is what you see in rules 6-8 in your screen-print. The column 'value 2' gets used only if the operator is BE but it's rarely used. The last 3 columns are basically just telling the rule where to print and how to print. Click F1 to open system help window, the explanation is quite easy to understand so you shouldn't have a problem.
Regards,
Thomas