What is "C1E support" on my AMD CPU?

More than just reducing the CPU multiplier goes on in C1E. The Processor to processor and processor to chipset links are also put into low power (high-Z) state. Allowing the whole platform to use less power but still appear to be on to the user.

From an AMD Blog:

C1E is a power management state that allows the processor to reduce power beyond just the cores. With C1E, the processor power can be reduced by lowering the memory controller clock speed and halting the HyperTransport™ technology links. This new feature was extremely important for our 12-core processors with the increase in memory channel support as well as the increase in HyperTransport™ technology links within the design.

According to AMD's publicly available BIOS and Kernel Developer's Guide, section 2.4.3.3:

The C1 enhanced state (C1E) is a stop-grant state supported by the processor. The C1E state is characterized by the following properties:

• All cores are in the halt (C1) state.
• The ACPI-defined P_LVL3 register has been accessed.
• The chipset has issued a STPCLK assertion message with the appropriate SMAF for C1E entry. Note that [The ACPI Power State Control Registers] F3x[84:80] specify the processor clocking and voltage behavior in response to the C1E SMAF.
• The processor has issued a STOP_GRANT message to the chipset.

General requirements for C1E:

• The ACPI-defined C2 and C3 states must not be declared to the operating system.
• C1E should only be enabled when the platform is in ACPI power management mode.

It automatically reduces your CPU multiplier (slows CPU down) when the computer isn't doing much. It's a way to save power.

And according to this page, your CPU supports it.