A resource is a kind of material or energy that is stored in certain parts, and can be generated or used by other parts. All resources available in a craft can be viewed at any time during a mission using the Resources button at the upper right of the screen. Common resources include liquid fuel and oxidizer, both of which are used in liquid engines, and electric charge, which has many different applications.
Transfer and crossfeed
Most resources can be transferred from one container to another part, which is usually done by right-clicking two containers while holding the mod key and then using the in and out buttons under the resource indicators. The transfer is stopped automatically once the source container is empty or the target container is full, but it can also be stopped manually using the stop button under the indicators. Multiple resources can be transferred at the same time, however the containers' right-click menus must stay open, otherwise the transfer will stop as well.
Another way to transfer resources is using a separate part. This is currently only possible for liquid fuel and oxidizer, using the FTX-2 External Fuel Duct.
Electric charge is the only resource which could be regenerated or refueled without starting another rocket. The only resources which can't be transferred at all are solid fuel and intake air. The latter doesn't have any sort of storage either, so it has to be produced when needed.
All means of resource storage are full when a craft is launched.