1 - If you look at the visual basic editor, you can see at the top of
the right hand side a box which may well say GENERAL to start with -
changing this to worksheet will allow you to see the events of the
workbook - these are things like open, change etc - so you can trigger
a macro when this event happens - obviously in your case, you want to
run something when a cell changes - in this case it's not important
WHICH cell, but you can pick that up with this event as well

2 - lcase turns the text into lower case - I've done this as YES, yes,
Yes and yES all read the same to you, but don't to VBA - so we can
cater for any form of the letters Y E S without spaces by turning the
value into a lower (or upper!) case string.