A line-level input is one that takes a pre-amplified signal, such as from a tape deck, a mixer or an electronic keyboard. Usually, this would not be sensitive enough for a microphone signal or an electric guitar or bass.
An audio interface would have microphone and/or line level inputs and usually outputs as well, and it would attach to your machine via USB, Firewire or install in the machine as a PCI card or a PCMCIA card.
http://acoustics.c.dictonarypage.co.uk/line+level/
Electrical audio signals within the range of -10 to +30 dBu. Usually in circuits with a 10k ohm load. Line levels are found on the outputs of tape decks, CD players, keyboards, mixers, most signal processing equipment, VCR's, etc. They can be balanced or unbalanced. Line level signals are much higher than microphone level signals.