This is a very light way to use "I believe". Usually people say "I believe..." when describing their important and strongly-held opinions. For example:

I believe that everything happens for a reason.

But you can also use "I believe..." to talk about something that you're not completely sure of. Here are some examples of "I believe" used this way:

There's a keyboard shortcut for that... I believe it's "Control" plus "E".

A: When is the company picnic?

B: I believe it's on the fifteenth.

When you use "believe" this way, emphasize the word "believe", rather than "I".