The New York Times explains it this way:
“I’m going to need you just as fired up as you were in 2008,” Mr. Obama told them.
When a heckler yelled “You’re a liar!” in response, people nearby spent the next few minutes shouting the heckler down, making it difficult for many in the audience to hear what the president, who was flanked on the dais by Maryland’s top elected Democrats, was saying.
Now see it happen:
From the YouTube post:
President Obama came to Bowie State University and was met by a student that spoke out against his speak concerning how conservatives and Republicans are the reason for today's economic condition. The student screamed out "you're a liar", causing the president to pause his speech. Then the student was physically beaten up by supporters, cursed down, and thrown side to side.
Obama then immediately changed his speech to start to defend himself and policies.