When you open an ME terminal, the mod scans the storage cells on that network. If remaining capacity for a resource type is critically low, a WARNING! dialog appears instead of opening the terminal right away.
You can ignore the warning once and continue into the terminal, or close it and free space first.
From any ME terminal, use the warning triangle button on the left toolbar to enable or disable capacity warnings per resource type (items, fluids, chemicals, and others). Choices are saved per player.