You are partially restoring groups of tables which belong together
In the example above, the user has chosen to only restore the languages table, and not to restore the tokens table, even though the 2 tables are linked logically (each token translation contains the reference to one of the languages).
The risk in is that the logical relation between data element be broken. In the above example, token translations entered for languages added since the date of the backup set selected will still be in the tokens table ... but there will be no way to access them since the language used to select them does not exist anymore (or rather yet, since we are restoring an earlier backup). Worse, when you do add a new language to the languages table, it will use the ID of one of the languages that was in the table that was overwritten, and you will suddenly see translations appear out of nowhere (the same translations that were "lost" immediately after restoring the languages table.
To summarize, you should take this message very seriously and, unless technical support suggests otherwise, you should always restore entire groups of logically related tables.
As shown above this means that the left-most checkboxes should always be fully checked or fully unchecked, but never half checked as was the case in the first example.