Triangulation is verifying the information by comparing it with other sources.
Information gathered during an interview is not always correct. Sometimes it is possible to check the information when compared with other sources.
Suppose a manager says during the interview that the turnover of the company last year was about € 500,000. This information can also be found in the yearly report. If this report reports the same amount of turnover, then the information from the manager during the interview seems to be correct.
If the turnover in the yearly report is € 1.000.000 something is not correct and might lead to doubt about the reliability of the manager or is the information in the yearly report incorrect? Now it is time to check it out or leave it this way if the exact amount is not really interesting for the research.