This validation checks if the translation contains required strings. Each string row can contains one or more required strings. They are used to check that translation contains the data that is necessary. For example you might have the name of the product in you string and you want to make sure that the translation contains the same name. Take a look at the following string: "SUPERCLEANER is very easy to use". SUPERCLEANER is the name of the product and if the company has decided that it must be always written like this the string should be set as required. When translator translates this is is very likely that they will translate it in a different way. For example Finnish translations might be "Supercleaner on todella helppo käyttää". Here the translator has followed the standard Finnish custom but it is against the company policy where name would always be written in capital letters. It should be "SUPERCLEANER on todella helppo käyttää". Japanese translations might be "スーパークリーナーはとても使いやすいです". This is according normal Japanese way but against company policy. It should be "SUPERCLEANERはとても使いやすいです".
To set a string required right click on the left side of the row and choose Row | Required Strings. Enter the required string(s) to the dialog that appears.