Voltage is a measure of the amount of EU a generator in Industrial Craft 2 will generate per-tick. Much like machines in the real world, the machines and cables in Industrial Craft 2 will only accept up to a certain level of voltage. If a machine is provided with a voltage higher than it can accept, it will explode and the machine and potentially the cables that connect it will be lost. The type of cable used to connect a generator to a machine must also match the voltage level of the setup. Each level is also significantly more dangerous than the last if the player comes into contact with a uninsulated wire carrying that specific kind of current. It's important to pay attention to the voltage of each machine or generator for this reason.
As an example of how voltages need to be matched in a system of machines, a Macerator is a Low Voltage machine and will not accept more than 32 EU/t. A Macerator can be connected to a BatBox (which outputs 32 EU/t) using Copper Cable (which can accept a maximum of 32 EU/t) without exploding. Even if one were to use Glass Fibre Cable instead, the system would still operate without exploding, as the Glass Fibre Cable can handle the voltage. On the other hand, Ultra-Low-Current Cable (which accepts up to 5 EU/t) cannot be used, as even though the Macerator and the BatBox can handle the specified voltage, the cable cannot.
Each tier of voltage also has a specific EU storage item for use with that voltage. Lower voltage items can be used to power machines of a higher voltage, but not vice versa; a RE-Battery (which is Low Voltage) can be used to charge an MFSU, but a Lapotron Crystal (which is High Voltage) can't be used to charge a BatBox.
The voltage of a machine is usually not marked in its GUI. The exception to this are the EU storage devices, which will have their maximum possible voltage, as well as their output voltage, marked in their GUI. Note that there is an EU storage device for each tier of voltage.
Low Voltage refers to voltage under 32 EU/t. This is the highest level of voltage that most Industrial Craft 2 machines will accept. Machines powered by Low Voltage generators cannot explode. On the other hand, the Ultra-Low-Current Cable cannot handle higher than 5 EU/t and will burn up. All other cables can accept Low Voltage.
Medium Voltage is the second tier of voltage and refers to voltage over 32 EU/t but under 128 EU/t. Most machines, with the exception of the Induction Furnace, the Mass Fabricator, the Terraformer, both the MV and the HV Charging Bench, and the Oil Fabricator will not accept a Medium Voltage charge without the addition of either a Transformer Upgrade in one of their upgrade slots or a LV Transformer connected to them. Similarly, Ultra-Low-Current Cable and Copper Cable (which can only handle Low Voltage) will burn up if connected to Medium Voltage. Gold, Glass Fibre, or HV Cable is required instead.
High Voltage is the third tier of voltage and refers to voltage over 128 EU/t but under 512 EU/t. Only the Mass Fabricator, the Terraformer, the Oil Fabricator, and the HV Charging Bench can accept High Voltage natively. To connect a machine to High Voltage, two Transformer Upgrades in the machine's upgrade slots or a MV Transformer connected to a LV Transformer is required. Along with the lower voltage cables, Gold Cable cannot be used to transport High Voltage and a Glass Fibre Cable or a HV Cable must be used instead.
Extreme Voltage refers to voltage over 512 EU/t. No machine, item, or cable (aside from HV Cable) can transport or will accept Extreme Voltage without a HV Transformer. The only generator that can produce Extreme Voltage is a Nuclear Reactor.