Why VoIP means digitizing audio, packetizing it, and shipping those packets across the internet instead of over a dedicated circuit. In ham radio this is what lets systems like EchoLink, IRLP and AllStar tie repeaters and individual stations together worldwide over internet links. The RF part is still normal amateur transmission at each end; the internet just carries the audio between the linked nodes.
Watch out The DX "spotting" choice describes DX cluster networks, which pass short text reports about who is on what frequency, not the voice audio itself.
VoIP = Voice Over Internet Protocol: voice turned into digital packets, exactly what the name says.