Conditional orders live on the broker's servers rather than at the exchange, which makes them flexible but slower and dependent on the broker's data and uptime. Nothing is visible to the market until the condition fires.
The condition can reference a different instrument, which is what makes them useful: hedges, pairs and index-triggered entries all become one instruction.
Example: "if the index future trades below 4,950, sell my 500 shares at market." Nothing rests in the book until the trigger, so no one can see your intention. The trade-off is trigger latency: the broker evaluates the rule on its own feed, so a fast move may execute you well past the level.
Related: one-triggers-other, otoco-order, market-if-touched