While most of the outcomes are gains of yardage, conservative plays can result in a loss of yardage and can occur only when both sides choose conservative plays. If you are ahead in the score you might tend to play more conservative on offense, which can result in slower steadier drives.
Playing conservative on defense gives up more plays of medium yardage but prevents an immediate touchdown. You can think of conservative defense as defensive backs keeping passing routes in front of them or defensive linemen staying in their lanes on running plays.