Every Cisco ASA platform comes with a certain number of implicitly activated features and capacities as a part of the Base License. In other words, these capabilities are fixed in the given software image for the particular hardware; you cannot selectively disable them. One example of such a feature is Active/Active failover, which is always available on all Cisco ASA 5585-X appliances. Some platforms offer the optional Security Plus license, which may unlock additional features or capacities on top of the Base License. For example, you can increase the maximum concurrent firewall connection count on the Cisco ASA 5505 from 10,000 to 25,000 by installing a Security Plus license.
Source and all optional licenses/features explained here.
Optional
is an additional license. Additional fee must be paid. Site-to-site VPN's do not require any additional licensing.