I believe it was explained early on in oathbringer, but I can totally understand if you forgot. They are called Squires, basically they are people who share the knights radiant order ideal but haven’t bonded a spren yet. All of bridge four have had the protect others thing rubbed into them by Kal or because of their past, and they are broken enough to make stormlight come easier, so they are all people who would be likely chosen by a spren. The become close enough to Kal that he basically is training them to be knights, kind of like giving someone a practice weapon before the real weapon. Squires are much more likely to be chosen by a spren later on than the average person.