The Nail Knot is what holds your leader to your fly line — it has to be thin enough to slip through rod guides without catching, and strong enough to survive thousands of casts. Once tied right, it lasts months.
Traditionally tied with a small nail (or a hollow tube) as a forming guide: the leader wraps 5–7 times around the nail and the fly line together, the tag end threads back through, and the wraps slide off as you tighten. Most modern fly anglers tie one leader connection per season and don't think about it.
Loop-to-loop systems (a small braided loop on the fly-line tip plus a perfection loop on the leader) are faster but bulkier. Anglers who care about clean turnover use the Nail Knot.