Specifically, held at open means trading for an exchange or equity occurs before the start of trading. Many companies will wait until after the market closes to issue sensitive information. For example, many companies will release earnings reports after the closing bell.
On the one hand, this gives investors time to digest the information and determine if it is significant. But in an age of 24/7 news coverage and high-speed trading, investors will often set up buy and sell orders to be executed when the market opens. This can lead to a large imbalance between buy and sell orders prior to the market opening.
When this occurs, an exchange may delay the opening of a specific equity until a balance between buy and sell orders is in place.