I thought
@arnelgp would reply. If you do not pre-define variables, you can still use them, but vba will create them as variant types. If you spelled variables differently in different places., your programme may not work correctly, and you may find it hard to work out why.
So after you add option explicit and then compile the application you my find that you need to declare variables that you have used without declaring them.
(note you can set this a standard in your app - in a module do tools/options/require variable declaration set to true)
You also need to be aware of the scope of a variable. A variable can be declared to have scope within the whole application, just one form, or even just one procedure, which again has an impact on your coding.
Your programme not compiling may also be to do with syntax errors in code, that you may not even use. It's not necessarily down to variable declaration.