Over most of Anderson's novel Speak, the protagonist, Melinda Sordino refuses to "speak." There are many times in the text where someone is speaking to Melinda and the text states her response as "Me:" with a blank after. Melinda has simply refused to say anything.
To understand this, one needs to understand why Melinda chose to silence her own voice. At a party, over the summer prior to Melinda's freshman year in high school, Melinda was raped. Given that she, in a heightened state of fear, called the police (she did not tell them she was raped). The party was broken up, everyone knows who called the police, and Melinda was alienated because of it.
Because of the way she is treated by her "friends," Melinda begins to pull away from everything in life. Not only does she lose her friends, her family life is less than supportive (she did not tell her parents either).
Therefore, the title of the novel is of the utmost importance based upon the fact that readers want Melinda to do one thing: SPEAK!