It's pretty self-explanatory, really. If you enable that option, Battery Saver will be turned on until the next time you charge your phone, at which point it will turn off automatically. While Battery Saver is enabled, non-essential background processes will be disabled, as will be push notifications, meaning you won't receive notifications for new emails or social network activity. Some live tiles might also stop updating. This helps conserve battery power, though at the cost of reduced functionality.
The "now until next charge" option is handy if, for example, you know you're going to be away from mains power for so long that even the "When battery is less than 20%" won't cut it. You can of course also select the "Always" option, but then you'll have to remember to change it again later to resume normal operation of your phone.
For more information on using Battery Saver, see the official how-to at WindowsPhone.com.