This one is also pretty self-explanatory. Default Group Policy behavior is to reprocess Preference settings during every Group Policy background refresh cycle. This means your preferences are redeploying and consuming processing resources on your machines every 90 minutes on average. Sometimes you will deploy settings that really only need to be changed once, and then never touched again. Rather than reprocess these preferences every time that group policy refreshes, selecting the box for Apply once and do not reapply will do just that, configuring the setting inside Windows once and then ignoring it in the future. This saves CPU cycles, but if the user contradicts the setting later by performing a manual change, their own configuration will be the one that gets left in place.