Create a drop down list which is made up of the other sheets

hi usrobinjo, welcome to the forum. assuming the data you want in "January" sheet is in A2:A10, you can try any of this 2 methods.

1) in "Product Line Summary", select the cells you want the dropdown applied to (say B1:B5). go to Data -> Data Validation -> Allow: List -> Source:
=January!A2:A10
OK

or;
2) name the range of the individual sheets first. go to "January" sheet & select A2:A10. press CTRL + F3 -> New -> Name: January -> OK. then in "Product Line Summary", select the cells you want the dropdown applied to (say B1:B5). go to Data -> Data Validation -> Allow: List -> Source:
=January

hope that helps