Modern CPUs are made using CMOS technology. MOS technology had two different types of transistors; N-type and P-type. They switch between being very conductive and very non-conductive when an electric field is present. The N-type switches on [conductive] when a positive voltage is applied. The P-type switches on [conductive] when a negative voltage is applied. When used together they "compliment" each other. The inputs are tied together and when voltage is applied the N-type conducts and the P-type doesn't. When ground voltage is applied the P-type conducts and the N-type doesn't. It's a push pull type of situation. Tri state
Push-pull is nice and all. Pulling electrons is a positive voltage and a digital '1'. Pushing electrons is a negative or ground voltage and is a digital '0'.* What if we have a bunch of registers that can all be access from the bus? They can't all be connected to the bus at one time. This is where a third state in digital logic systems comes in. It's called High-Z [high impedance] or floating output. This is where the output is disconnected. It no longer pushes or pulls.
* It should be noted that electron flow and current flow are the opposite of each other. This dates back to Benjamin Franklin. He arbitrarily decided that current flows from positive"+" to negative"-". Electrons, as we know, are negatively charged "-". They are more concentrated at a negative potential [source, emitter, cathode] and flow into the empty space found at a positive potential [sink, collector, anode]. So don't get confused when dealing with transistors because we think in terms of electron flow and not current flow.