Line-level is the standard operating level for audio gear connections: DAC to amplifier, mixer to recorder, interface outputs to monitor inputs. Professional gear uses +4 dBu (1.228 volts RMS), consumer gear uses −10 dBV (0.316 volts RMS). This is why connecting consumer gear to professional inputs results in low levels, and pro outputs to consumer inputs can cause distortion.
Microphone level (a few millivolts) must be amplified to line level by a preamp before recording. Instrument level (guitar/bass pickups, ~100mV) needs a Hi-Z input or DI box. Speaker level (tens of volts from an amplifier) should NEVER be connected to line-level inputs — it will destroy them.