[Question]Can someone explain the context_freelists?

In MemoryContext source code:

/* context_freelists[0] is for default params, [1] for small params */
static AllocSetFreeList context_freelists[2] =
{
    {
        0, NULL
    },
    {
        0, NULL
    }
};

I really don't know what this variable mean, can someone share me with your view?
Thanks in advance!

Yours,
Wen Yi