How to interpret wireless performance?

Hi again,

I just got the new Airport Extreme setup with 2 Airport Expresses. The AX are set to join existing network which is hosted by the AEBS in the 5 GHz range.

Can someone explain how to read the performance graph in the Airport Utility? It's not documented in the help section. I have no idea how to interpret the numbers. I'm assuming the dips in the graph are not a good thing.

The yellow line is the AX located right above the room where the AEBS is. The green line is the AX located in the room right beside it.