In his death throes, Mercutio says this after being stabbed by Tybalt. He is cursing both the Capulets and the Montagues - that is the "plague o' both your houses" he refers to. Had it not been for their feuding, he would not be dying.
"I am sped" reflects his awareness that he is dying.
The last question refers to Tybalt. He wants to know if he has run away, and if he was hurt at all. I suppose it makes it more bitter for him to have been killed without striking a blow in defense. Of course, the fatal blow was made possible by Romeo's stepping between Tybalt and Mercutio, which sets up Romeo's next move of killing Tybalt out of guilt and revenge.
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