Dial-up describes an old way of sending information from one computer to another over a telephone line. The computer used a modem and literally rang a number, the way you used to dial a phone. While it was connected, that same line often could not be used for a voice call; it was slow, and it made a loud screeching sound. This was ordinary home internet in the 1990s and early 2000s. Broadband — a line that stays on — replaced it. People still say “dial-up” as a joke when the internet feels painfully slow, even when no phone is being dialled. You will see it as an adjective (a dial-up modem, a dial-up connection) and as a noun (we were still on dial-up).