A schematic is a simplified diagram of a complex object that acts as a guide to building that object. In Astroneer the schematics are represented by pale blue (almost clear) outlines of complex objects than can be made by players. Other names players may use for this include: blueprint or recipe.
In Astroneer, players can see which schematics they have by using the forward and back arrows while the backpack open or while standing at the a camp module like the printer. As you "page through" a visual representation of each schematic will appear. For example, you may see the schematic for a solar panel, wind vane, or storage unit at a printer or at the Vehicle Bay some basic vehicles, like the Rover.
Some schematics must be discovered through a process of scavenging and subsequent research. For example, the filter schematic must be discovered before it appears in the backpack's options and more complex vehicles like the Truck must be discovered before they appear in the Vehicle Bay.