Special part types
In Flux, every part has a part type associated with it. This can be a resistor, capacitor, or one of several other part types to choose from. This is mostly used for categorizing parts, however some of these part types have special properties that are associated with them. Two of these are the "Ground" type and the "Portal" type.
Portal part type
Portals allow connecting components without wires, giving your design a cleaner and more organized look. A simple example part of a portal can be found here.
To connect multiple portals together, type in the same designator for each one. Every portal provided with that designator will be connected to the PCB and simulation. If two or more portals are connected properly, you should see all of the connected portals turn blue, as shown here:

Ground part type
Ground can be thought as a specific use case of portals. It is the common path for current in a circuit to return through. Similarly to portals, grounds prevent having to connect multiple wires together and keep the schematic organized. The unique part about the ground part type is that it supports ground fills, where a layer of the PCB acts as a net.
You can see in this image that there are several pads (shown in pink) that are directly connected to the top copper layer (shown in red). These pads are all ground type parts that have been connected to the layer itself. This ground fill is done via the "Connected Layer" rule in the ground net of the project.