what [] means?

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From: A.M (IHateSpam_at_sapm123.com)
Date: 04/14/04 

Date: Wed, 14 Apr 2004 09:53:00 -0400

Hi,

I am analysing a VB.NET code and i see this line of code:
Dim strCacheKey As String = [ProviderType] & "provider"

What is the role of [ ] ?

Thanks,

Allan