We often add 'up' to a verb to make the sentence more complete, especially if there is no object. For example, if the manager of a shop wants to ask their deputy to open the shop the next day, they might just say 'Can you open up tomorrow?'. There's no need to say 'the shop', and making a phrasal verb out of 'open' and 'up' gives the sentence a more complete verb. In a figurative sense, 'open up' can mean to start talking openly about one's feelings. For example 'After we'd been sitting comfortably for a while and had a glass of wine, John began to open up.' Those are just two examples. Here are a few dozen more idiomatic ones: http://idioms.thefreedictionary.com/open+up

Open up is an idiom in English which means to talk freely or without inhibitions. For example, 'Sam asked Emily to open up on stage to give an exhilarating performance. ' Open can be used as follows; Adjective: not covered, exposed. Noun: outdoors, in the field with sky above. Verb: to spread out, unfold. For example: She left the file folder open after borrowing the sheets from it. Let's go and play in the open. As she opened up the casserole the smell of fresh cake spread in the air and every body's mouth watered. P.S. Last one is my personal favorite. ????

Sometimes they mean the same thing. I open/open up a package. I open a door or a window, never open up. An idiom is. "He really opened up when she asked him about it". Which is to say, he really shared a lot of personal information.