"Merge logs" are VSys One's memory of your manually combining things like skills, jobs, restrictions, etc. together. If after a past database merge you combined the skills "Music training" and "Music instruction", VSys remembers this - but that's in the database itself, and the database may be overwritten when you do your next database merge.
When you've finished making any such changes after merging databases, come back here and click on Save merge logs. VSys will store those logs to a disk file. Next time you merge databases, if your target database was overwritten with new data and those merge logs are forgotten by VSys, click on Load saved merge logs to bring them back in. When VSys combines the databases, if it sees "Music training" and "Music instruction" it will automatically merge them together for you based on those past logs.
For simplification, in the example above we used the text description of the two skills. VSys does not really remember the merging of values by their descriptions. Instead VSys uses the underlying unique ID code for each skill.