If you are not able to protect your account with a Steam Guard Mobile Authenticator, a hold will temporarily be placed on items moving away from your account. Trade and market holds protect items if a user's account is compromised. These holds also give users a way to recover items before they are lost. Even if a hijacker manages to access your account, you can prevent them from stealing your items by canceling any transactions that are on 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.
A market hold is a period of time after a sell listing is created where the items are held by Steam before they are available on the Community Market.
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.
For more information, please review this news post
If you haven't had your account protected by a Mobile Authenticator for at least the last 7 days, items leaving your account will be held by Steam for up to 15 days. This provides you time to discover any transactions on hold that you didn't make. If you find that you account has been compromised, you can change your Steam account password and cancel any item transactions that are pending before your items are stolen.
When you cancel a trade that is being held by Steam, we'll presume that you intercepted a hijacker and you'll see a 7 day trading cooldown to prevent any further unauthorized attempts to trade away items. This cooldown is for your protection and cannot be removed by Steam Support.
Trade and market holds are no longer necessary when you protect your account with a Steam Guard Mobile Authenticator. Using a physical device (your smartphone) to confirm item transactions ensures that you and only you can quickly and securely trade or sell your items. You can get the Steam Guard Mobile Authenticator for iOS, Android, and Windows Phone devices. Items will no longer be held after you have had the authenticator enabled for 7 days.