What do I mean by cross referencing?
If you are undertaking an action that has a constraint on it elsewhere in the rulebook, you need to cross reference the section that talks about the constraint while you explain the action. For example, imagine a board game where you are only allowed to take three action movement in a round. This constraint is started at the very start of the rule explanation under the title Action Movement Constraint. Now imagine you have a section of the rules on how to execute the different actions. Say one of the actions you can do is pick an apple. A cross reference will say that the picking an apple action counts as one of the three action movements mentioned at the start of the rule book under the heading Action Movement Constraint. If there was no mention in the section that picking an apple cost one of your three actions in the pick the apple section, then there is no cross reference.
Why is cross referencing Important?
What I observed in the blind playtest is if there was no cross reference to a constraint on an action said elsewhere in the rule book, players learning the game assumed away the constraint.
Take a trucking game with a trailer component. The trailer has little squares where you place the cargo it can carry. Assume that the rules mention that each space can only hold one cargo in the game component section. However, assume a page or two latter there is a section on loading the truck. If the section on loading the truck does not cross reference the earlier mention that the truck only has room for one cargo in each space, then some players will not recall that was mentioned earlier in the rules and will be unclear on how many goods a truck can carry.
I observed in the blind playtesting that even when players were lucky enough to connect the dots without the rule being cross referenced, there was a large delay in learning the game. The players had to pause, think, and flip back in the rule book to the section they thought they remembered had the important rule.
Cross referencing in board game rules gets rid of that down time for lucky players and prevents other players from leaving a rule out all together.