What is the meaning of "greasing the pan"?

The metaphor is one of preparation. The act of "greasing the pan" has no real product; the result is simply a greased pan, which is useful only as a means to an end and is not an end in itself. However, it is a necessary step to perform prior to "pouring in the batter", which would generally be seen as a productive step (especially if the pan is pre-heated, as in a skillet for making pancakes).

Other metaphors include:

• Laying the foundation - performing necessary "ground work" to provide a solid base on which to build something substantial. The results of the work, again, are not an end in themselves, and most of this work won't even be visible when the project is complete, but it is very necessary.
• Making the bed - This one typically has a negative connotation: "you've made your bed, now lie in it", implying that the subject has, by action or inaction, done something that has in turn resulted in unpleasant consequences for them.
• Planting the seed - This one has the metaphorical meanings of doing something seemingly very small that will over time "grow" to result in something much larger, or of performing an action whose immediate results aren't apparent but will be later. The connotation though is usually one of needing only cursory care or maintenance after the initial action.
• Setting the stage - Thanks to JAM for that one; there's a lot of "behind the scenes" preparation that simply must happen to put on a stage production, including building and installing the scenery the players will be performing in. All the audience sees, however, is the end result when the curtain rises.

We've greased the pan = we've gotten everything ready.

Now it's time to pour in the batter = now we're going to do the work/job/project.

Another expression (far more common) is to set the stage: To prepare; to establish the basis or required conditions.