By default, any member who renews his membership will see his membership start the day after his current membership expires.
The "automatic activation" option allows you to define which membership type(s) will not apply this default behavior. To set this up, go to the "Membership Types" management screen. Access this menu in the Members application "Settings" :
Choose a membership type from the list to modify its configuration. Select the "Rate" tab.
In the Behavior of renewals section, check the automatic activation option for renewal.
This way, if a member has a "Standard Membership" and wishes to renew it, his new membership will take effect immediately, without waiting for the expiration date of the current membership.