AnyConnect Essentials is an old term and no longer offered as a product.
The AnyConnect Plus licenses include everything that was included in Essentials (and a few other things like what used to be separately sold as AnyConnect for Mobile).
The other tier is Anyconnect Apex (roughly equivalent to the old Premium license). it adds features like posture assessment (from the ASA or via ISE), clientless SSL VPN, and other advanced features.
Both Plus and Apex licenses are offered as term licenses (1, 3 or 5 years). You can also buy a perpetual license for Plus - it it priced somewhat more than the 5 year term license.
Lots more details can be found in the Anyconnect Licensing FAQ here:
https://www.cisco.com/c/en/us/support/docs/security/anyconnect-secure-mobility-client/200191-AnyConnect-Licensing-Frequently-Asked-Qu.html
