combo boxes select item from first and then have choices for that item show up auto in the second combo box

Hello,
 I want to have two combo boxes on a form in access. I want to be able to select an item from the first combo box and then when go to select an item from the second combo box - only the items relating to the first combo boxes  selected item will show up. I also want both boxes by default empty.
I have found an example online - its what I want - but don't understand- I never made a table where the first field had a plus sign - which if you click - other fields drop down.
heres some stuff from the example to show you-
query:
SELECT tblStore.strStoreName, tblManager.strManagerName
FROM tblStore INNER JOIN tblManager ON tblStore.lngStoreID = tblManager.lngStoreID
GROUP BY tblStore.strStoreName, tblManager.strManagerName;
table1
lngManagerID      lngStoreID      strManagerName
1      1      John Smith
2      1      Lee Thomas
3      1      Alison Jones
4      2      Tim O'Brian
5      2      Simon Marsh
6      3      Harry Hill
7      3      Sally Lees
8      4      Jenny Parker
9      4      Ian Jennings
10      4      Fred Lee
11      4      Bill Hardy
12      4      Alan Parker
table2
lngStoreID      strStoreName
1      M&S
2      Binns
3      Safeway
4      B&Q
5      

when click (+) sign on table 2 -  records from ingManagerID and strManagerName open up

Hope you can understand my question and can help me.
Thank you