What does lock taskbar mean

[madukes36]

my taskbar is double the size it was, how do I get it back to smaller width

 

[ukcal]

Right click an empty space of the taskbar, uncheck "Lock the taskbar". Hover the mouse over the top edge of the taskbar and the mouse should become the up and down arrow. Click and hold and then drag back down. Recheck "lock the taskbar" to stop you from accidentally making it larger again.