An Order Cancellation Line allows you to draw any of the 4 valid straight lines on your chart and set a specific value in its Tag to enable it as an Order Cancellation Line. If price hits that line, 

all open orders waiting in the market will be cancelled if you are not currently in any positions for that instrument. Read about valid straight lines.

Read about setting up Order Cancellation Lines here.

IMPORTANT: If you are currently in a position on that chart and price hits an Order Cancellation Line, nothing will happen.  This is to make sure your open Target or Stop orders are never cancelled.

Read the details about what actually happens when an Order Cancellation Line is hit by price.