What does the author mean when he refers to journalists in sourcing as "professional gatekeeping"?

The author meant that "professional gatekeeping" refers to the filtering out of information to the public according to their veracity and this is what journalists do.

Explanation:

The question above is related to the article entitled, "Social Media Users Must Start Checking Online News Sources." It centers on the importance of checking the veracity or reliability of news due to the fact that a surmounting fake news circulate on the internet.

The author then mentioned the phrase "professional gatekeeping," which is the most overlooked situation for online readers. Readers have to make sure to check the journal or article that they read according to the credibility of their sources. In this way, they'll fully understand the importance of "journalistic sourcing" (as mentioned in the text).

So, this explains the answer.