LTC stands for Longitudinal Time Code. This is a audio signal which transmits a series of square waves from a "Master" device. The Master device would be the main device you are running the timecode from, this could be from mixer to camera or separate timecode box to a mixer or camera.
The device that is being synced with the "Master" device is known as the "Slave". These are common terms to which anything is in charge of something else. Or instance in other timecode modes you can trigger an audio recorder to go into record mode from a camera going into record mode using a pulse of timecode.
LTC is an audio signal so is processed differently in post production than metadata which SMPTE Timecode is. There are plugin and software you can get to read and match the LTC timecode data. This is most commonly used on DSLR shoots where no SMPTE timecode input or output exists.
The other time code method the SMPTE adopted along with LTC in the 1960's was VITC, click here to read more.