What does "I do believe" mean? [duplicate]

Sentences like "I do believe" or "I do love you" are common. But I was wondering what do they mean? I've never seen structures like this in grammar.

In fact I don't understand what is the diference between these sentences and ,say ,this simple one: "I believe" or "I love you"

Thanks in advance.


The primary verb do is used in this way to emphasise the main verb.