Tetra is simply short for tetraploid. This means each cell has four sets of chromosomes instead of two, the normal number. This makes for physically stronger plants and blooms having more substance. It also increases fertility in crosses that would otherwise be difficult.
The fact that White Amber is tetraploid will not be very important to you unless you are planning to do some hybridizing, in which case the information is important. You would use it with other trumpet or orienpet lilies that are also tetraploid.