Wise commanders balance all metal produced into metal expenditure, but it's always nice to have a little emergency fund (60% of commanders don't have enough savings to respond to a Krow rush!).
The resource bars represent how much metal and energy is stored for future projects. While there is no direct button to set a reserve storage, it is possible to reserve part of your maximum storage capacity to not be used for low and normal priority construction by Ctrl-left-clicking on the bars. When reserve storage is set, whenever you hold fewer resources than the set reserve, low and normal priority construction will pause until your storage fills up. Anything set to high priority will ignore set reserve limits. This allows a commander to push more build power into a unit or structure than income would permit, until storage runs out.
You can set a pre-game percentage of your storage to be reserved by Space-clicking on the economy bars or by navigating through the in-game menu to Settings/HUD Panels/Economy Panel and changing the Reserve settings.