Here's the full prophecy:
"The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not... and either must live or die at the hand of the other for neither can live while the other survives... the one with the power to vanquish the Dark Lord will be born as the seventh month dies..."
Voldemort only hears the first part of the prophecy:
"The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies..."
This is why he chooses to target Harry. He believes that Harry is the one who will be able to defeat him, so he wants to kill him to prevent that from happening.
Voldemort never learns about the later part of the prophecy, which states that Harry has power he does not know, and that one of them must die. This is crucial because it reveals that Harry is not merely a threat but a necessary component in his own demise.