Ben (pronounced like bain) is a very common filler word or interjection with various meanings; you could write an entire linguistics paper about it (seriously, someone did). In most cases, it can be translated as “well”.
One of its most common usages is to indicate that the speaker thinks the following statement is obvious.
— T’as fait les courses ?
— Ben oui, tu vois bien que le frigo est plein.
— Did you go shopping?
— Of course, you can see the fridge is full.
In other cases, it indicates an hesitation.
— T’étais où lundi soir ?
— Ben, euh… je sais plus.
— Where were you on Monday night?
— Well, uh… I don’t remember.
It can also indicate an explanation.
— T’as déjà vu ce film ?
— Ben en fait, je voulais aller le voir, mais je suis tombé malade.
— Have you seen this movie?
— Well, actually, I wanted to see it, but I got sick.
As you may have noticed, it usually occurs in dialogues, when it is your turn to speak. You cannot just come to someone and start a sentence with ben.
There is a more colloquial variant, bah (the final h is useless), but it’s not very polite.
Listen to the examples: