It means AMD's 64-bit Athlon processor, Processor 300 is the model, equivalent to Intel's Ben 3 and Ben 4.
Central Processing Unit (CPU): AMD Athlon(tm) 64 Processor 300
Specifications: AMD Athlon(tm) 64 300
Kernel codename: Venice
Kernel version (step): DH-E6
Process: 90 nm
Main frequency: 1.81 GHz
Frequency multiplier: x 9.0
FSB: 200 MHz
HyperTransport bus speed: 1000 MHz
Level 1 data cache: 64 KB, 2-Way, 64 byte lines
Level 1 instruction cache: 64 KB, 2-Way, 64 byte lines
L2 cache: 512 KB, 16-Way, 64 byte lines
Socket/Slot: Socket 939
Motherboard Maximum supported CPU frequency: 3.00 GHz
Voltage: 1.7 V
CPU fan speed: 2766 rpm
CPU temperature: 25 ℃ (77 ℉ )
CPU core temperature: 39 ℃ (102 ℉)
Number of cores: 1
Number of logical processors: 1
Processor version: Family 15 Model 15 Stepping 2
BrandID: 4
Extended version: Ext.Family 15; Ext.Model 47
Technical characteristics
p>Intel MMX: Supported
Intel SSE: Supported
Intel SSE2: Supported
Intel SSE3: Supported
Intel SSSE3: Not supported
AMD MMX+: Supported
AMD 3DNow!: Supported
AMD 3DNow!+: Supported
Hyper-Threading Technology (HTT): Not supported
AMD CMP Legacy (Multi-core technology): Not supported
IA-64 Processor Capable: Not supported
Intel EM64T(64 Bit extended memory): Not supported
AMD X86-64 Long Mode: Supported
Built-in floating point coprocessor (FPU): Supported
Virtual Mode Extensions (VME): Supported
Debug Mode Extensions (DE): Supported
Page Size Extensions (PSE): Supported
Time Stamp Counter (TSC) : Supported
MSR register read and write (MSR): Supported
Physical Address Extension (PAE): Supported
Computer Exception Check (MCE): Supported
p>Compare and Exchange Command (CX8): Supported
Built-in local APIC (APIC): Supported
Fast System Access (SEP): Supported
< p> Memory Range Register (MTRR): SupportedPage Global Enablement (PGE): Supported
Processor Check Architecture (MCA): Supported
Conditional Command Transfer (CMOV): Supported
Page Attribute Table (PAT): Supported
36-bit Virtual Memory Extension (PSE-36): Supported
Fast Floating Point Save and Restore (FXSR): Supported
Enhanced Intel SpeedStep: Not supported
Thermal Monitor 2: Not supported
LAHF/SAHF commands (64-bit submode): Supported
Intel Virtual Machine Extensions: Not supported