The kernel log buffer is simply a memory buffer within a kernel address space where the printk output is saved (logged). More technically, it's the global __log_buf[] variable. Its definition in the kernel source is as follows:
kernel/printk/printk.c:
#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
It's architected as a ring buffer; it has a finite size (__LOG_BUF_LEN bytes), and once it's full, it gets overwritten from byte zero. Hence, it's called a "ring" or circular, buffer). Here, we can see that the size is based on the Kconfig variable CONFIG_LOG_BUF_SHIFT (1 << n in C implies 2^n). This value is shown and can be overridden as part of the kernel (menu)config here: General Setup > Kernel log buffer size.
It's an integer value with a range of 12 - 25 (we can always search init/Kconfig...
