By default, all qubits are initialized in the 0|0\rangle state. With the prep_x instruction, qubits will be initialized in the x-basis in the +=0+12|+\rangle =\frac{\vert 0 \rangle + \vert 1 \rangle}{\sqrt{2}} state:

State initialization can be done at the beginning of an algorithm, but it can also be done during an algorithm (re-initialization). Be aware that re-initialization of a qubit will also reset the binary register entry for that qubit to zero.

Bloch sphere courtesy of