An open roll is a roll that, while typically a random integer in the range of 1 to 100, has a 5% chance of adding another 1 to 100 roll or a 5% chance of subtracting the roll. The additional roll could be added again, any number of times, if the subsequent rolls are also 96 or above (or 5 and below, for a "negative" roll). Open rolls are used in foraging, Combat Maneuvers, and Picking Locks, among a few other systems.
It is possible to get infinitely high (or low) numbers in an open roll, however, the higher or lower the number, the more rare the odds of that number occurring. A roll above 100 (or below 0), for example, has about a 5% chance of occurring. A roll above 200 (or below -100) has less than a 0.25% chance of occurring, and a roll above 300 (or below -200) has less than a 0.0125% chance of occurring.
The concept of an open roll originates from when the world of GemStone was a part of the RoleMaster system (See ICE age).