At some point in time when you learn to crochet, you will need to know how to attach a button to a crocheted item.This can be used as a closure for wallets, bags or wearables. It is also frequently used as eyes or noses on character beanies and other amigurumi items.

I think the trick to securing it is tying the knot before weaving in the ends.

To learn how to weave in ends, see tutorial here