Attention is the requirement of a large amount of undivided attention to do something.
An example is airlifting reinforcements into a battlefield. This requires attention to not lose a large amount of troops while the transports are landing their loads.
There are three well-known resources: energy, mass and time. Building a unit has the obvious requirements of energy and mass. It also takes time, which is calculated based upon the unit being built and the capabilities of the unit(s) constructing it. Attention is a fourth resource.
For instance, the resources required to build a mobile missile launcher are similar to those required for other tech 2 land units. It is a very effective unit for destroying tech 1 defenses, but it must be carefully managed to keep it from moving within range of enemy units. They are not particularly expensive in terms of energy, mass and time, but they can be very expensive in terms of attention.