I just meant an audio clock source. An audio interface would be a piece of hardware that has it's own clock. If you just plug directly in to your PC, it will usually have sync issues, depending on the sound card and possibly even depending on the software you use to record. Back in the day, I used to plug my guitar directly into the mic input on the sound card, and I used Audacity to record, and there were sync issues. It wasn't super bad, but it made it impossible to get anything near professional sounding (not to mention it made my guitar tone sound very crappy).
The sync issues become very apparent when you try to record another track along with your previously recorded one. This never happens when you go through an audio interface, but it is very common with most standard sound cards.