In one sentence: Boosts are proactive while Auto Boosts are reactive.
You can top up your data manually by using a Boost. Just go to your CirclesCare > Dashboard and select the amount you want to Boost your plan with. The amount will be instantly charged on your credit / debit card on file.
If you activate Auto Boosts (CirclesCare > Menu > Settings > Use Auto Boost), every time you are close to running out of data, we will automatically boost your Data by 100 MB. The amount will be charged at the end of the month.