What is the meaning of this sentence "Let me be real for a second"?

Solution 1:

It means step out of the usual social politeness conventions, and be bluntly realistic.

http://onlineslangdictionary.com/meaning-definition-of/be-real

It includes a warning that what the person is about to say might be unusually blunt, might hurt the listener's feelings or surprise them.

Examples of possible next sentences after the sentence referenced in the question:

Let me be real for a second.

• That outfit really isn't doing you any favors
• This party is boring and your guests are going to start leaving unless something is done
• What you said last Tuesday really bothered me, and I need to know what you were referring to
• In high school, most of the time I told you I was in the library, that wasn't where I was.