It forces you to explicitly declare all variables.

What is the difference between explicitly declaring and implicitly declaring a variable?

Explicitly declaring a variable:

Dim anInteger As Integer = 1234

Implicitly declaring a variable:

'Did not declare aNumber using Dim
aNumber = 1234

Conclusion

Therefore, you should always have Option Explicit On as you could misspel a variable during assignment, which cause your program to behave unexpectedly.

Document level

It is on by default, but you can have an extra layer of protection by placing Option Explicit On at the top of the code file. The option will apply to the whole document.

Project level

You can switch it on via the menu in Visual Studio:

Project > [Project] Properties > Compile Tab > Option Explicit

Choose On in the drop-down menu. The option will apply to the whole document.

All new projects

You can switch it On by default for all new Projects by selecting:

Tools > Options > Projects and Solutions > VB defaults > Option Explicit

Choose On in the drop-down menu.

Remarks

Option Explicit On is a recommended good practice with Visual Basic .Net. It helps you as the developer to produce cleaner, more stable, more bug-free, more maintainable code. In some cases it may also help you write programs with better performance too!

with ref to https://support.microsoft.com/en-in/kb/311329#bookmark-3 (opens new window) option strict is also can be used instead of option explicit. Option strict inherits option explicit.