McCain had a miserable 2013 campaign and was cut by the Texans after the first year of a 3-year/$5.25M deal he signed with Houston last offseason. McCain put in his worst season since 2010, registering only 1 interception, 7 passes defended, and 32 combined tackles.
McCain played his entire career in Houston after being drafted in the 6th round of the 2009 NFL draft. The 27-year old 5'9" corner is mostly used against the slot receiver and on special teams.
The Steelers needed serious depth at the cornerback position - and still do - so I wouldn't expect this move to alter their draft plans. Darqueze Dennard and Justin Gilbert (not gonna happen) are still projected as top targets in the first round, as noted by reported Dale Lolley.
McCain's deal is likely to be for one year, and depending on the draft there is no guarantee that he will make it out of camp or suit up most Sundays. This story is most significant because it goes to show that the Steelers are always looking for new talent and moves that will surprise their fans, particularly in this whirlwind off season.