"Not held" means the broker is not held to the market. A human or an algorithm may wait, work the order over hours, or pass on a price entirely, using judgement about market-impact.
The client gives up the right to complain that a trade "should" have been done at a level that traded. In exchange, large orders avoid moving the market against themselves.
Example: a fund gives a broker 800,000 shares not held, in a stock that trades two million a day. The broker works 10% of volume across the session and prints an average within a few cents of vwap. A held-order for the same size at market would likely move the price several percent.
Related: held-order, participation-rate, algorithmic-order