1.5 What is a Trade Hold?

A trade hold is a period of time after a trade is accepted by both parties where the items traded are held by Steam before they are delivered. Trade holds last up to 15 days.

If you're trading items with a friend, and you've been friends for more than 1 year, the hold is 1 day. 

Holds protect your items. Steam accounts are valuable, especially if they have items, and that makes them appealing to thieves. Anyone participating in trading or the Community Market should have the highest level of security on their account.

If you haven't protected your account with a physical device (using the Steam Guard Mobile Authenticator), a hold will give you protection in the form of additional time to discover your account has been compromised and to prevent items from leaving your account.