Jesus faced a lot of hardship on the cross (AKA how Jesus got executed). While Jesus was left to die on a cross, the soldiers took Jesus's garments and tunic, split them up, and casted lots. In other words, Jesus had to die with barely any clothing on.
John noted that the garment scene was a fulfillment of past Scripture -- in this case Psalm 22:18. In Psalm 22, David talked about how he faced big afflictions from enemies around him. According to Merriam-Webster, "fulfill" can mean "measure up to" or "to put to effect", so in context John refers to how attributes on the past are a greater descriptor of the present scenario (basically how history repeats itself). In particular, the same attitudes that got David's enemies to cast lots on David's clothing were present when the soldiers casted lots on Jesus's garments. Christopher Smith went into more detail on what fulfillment is in this article of the Good Question Blog.