What Is List In Program

Hello
This code is source file of list. generally List means the number of items. I want to understand what is use of this program ? why do we need this source file ?

Code:

#include <stdlib.h>
#include "FreeRTOS.h"
#include "list.h"

void vListInitialise( xList * const pxList )
{
    /* The list structure contains a list item which is used to mark the
    end of the list.  To initialise the list the list end is inserted
    as the only list entry. */
    pxList->pxIndex = ( xListItem * ) &( pxList->xListEnd );            /*lint !e826 !e740 The mini list structure is used as the list end to save RAM.  This is checked and valid. */

    /* The list end value is the highest possible value in the list to
    ensure it remains at the end of the list. */
    pxList->xListEnd.xItemValue = portMAX_DELAY;

    /* The list end next and previous pointers point to itself so we know
    when the list is empty. */
    pxList->xListEnd.pxNext = ( xListItem * ) &( pxList->xListEnd );    /*lint !e826 !e740 The mini list structure is used as the list end to save RAM.  This is checked and valid. */
    pxList->xListEnd.pxPrevious = ( xListItem * ) &( pxList->xListEnd );/*lint !e826 !e740 The mini list structure is used as the list end to save RAM.  This is checked and valid. */

    pxList->uxNumberOfItems = ( unsigned portBASE_TYPE ) 0U;
}
/*-----------------------------------------------------------*/

void vListInitialiseItem( xListItem * const pxItem )
{
    /* Make sure the list item is not recorded as being on a list. */
    pxItem->pvContainer = NULL;
}
/*-----------------------------------------------------------*/

void vListInsertEnd( xList * const pxList, xListItem * const pxNewListItem )
{
xListItem * pxIndex;

    /* Insert a new list item into pxList, but rather than sort the list,
    makes the new list item the last item to be removed by a call to
    pvListGetOwnerOfNextEntry. */
    pxIndex = pxList->pxIndex;

    pxNewListItem->pxNext = pxIndex;
    pxNewListItem->pxPrevious = pxIndex->pxPrevious;
    pxIndex->pxPrevious->pxNext = pxNewListItem;
    pxIndex->pxPrevious = pxNewListItem;

    /* Remember which list the item is in. */
    pxNewListItem->pvContainer = ( void * ) pxList;

    ( pxList->uxNumberOfItems )++;
}
/*-----------------------------------------------------------*/

void vListInsert( xList * const pxList, xListItem * const pxNewListItem )
{
xListItem *pxIterator;
portTickType xValueOfInsertion;

    /* Insert the new list item into the list, sorted in ulListItem order. */
    xValueOfInsertion = pxNewListItem->xItemValue;

    if( xValueOfInsertion == portMAX_DELAY )
    {
        pxIterator = pxList->xListEnd.pxPrevious;
    }
    else
    {

        for( pxIterator = ( xListItem * ) &( pxList->xListEnd ); pxIterator->pxNext->xItemValue <= xValueOfInsertion; pxIterator = pxIterator->pxNext ) /*lint !e826 !e740 The mini list structure is used as the list end to save RAM.  This is checked and valid. */
        {
            /* There is nothing to do here, we are just iterating to the
            wanted insertion position. */
        }
    }

    pxNewListItem->pxNext = pxIterator->pxNext;
    pxNewListItem->pxNext->pxPrevious = pxNewListItem;
    pxNewListItem->pxPrevious = pxIterator;
    pxIterator->pxNext = pxNewListItem;

    /* Remember which list the item is in.  This allows fast removal of the
    item later. */
    pxNewListItem->pvContainer = ( void * ) pxList;

    ( pxList->uxNumberOfItems )++;
}
/*-----------------------------------------------------------*/

unsigned portBASE_TYPE uxListRemove( xListItem * const pxItemToRemove )
{
xList * pxList;

    pxItemToRemove->pxNext->pxPrevious = pxItemToRemove->pxPrevious;
    pxItemToRemove->pxPrevious->pxNext = pxItemToRemove->pxNext;

    /* The list item knows which list it is in.  Obtain the list from the list
    item. */
    pxList = ( xList * ) pxItemToRemove->pvContainer;

    /* Make sure the index is left pointing to a valid item. */
    if( pxList->pxIndex == pxItemToRemove )
    {
        pxList->pxIndex = pxItemToRemove->pxPrevious;
    }

    pxItemToRemove->pvContainer = NULL;
    ( pxList->uxNumberOfItems )--;

    return pxList->uxNumberOfItems;
}