Why should you be afraid of a man behind bars like that?句子中的

Why should you be afraid of a man behind bars like that?句子中的bars是什么意思?
I do not know how to combat this weakness.I have thought of just handing my money,taking the ticket,and then dashing off with a cheery:“ The rest is for you!” to the man.I have tried saying to myself,as I stood in line:“ Why should you be afraid of a man behind bars like that?He is probably just as afraid of you." This is not true and I know it.) I have tried pushing my whole hand under the bars and holding it open for him to drop the change into,but then I find that,as I close my fist on it,I am unable to get my hand out again.
这是课文中的一整段