"multiplier-locked" means it's not a -k series processor, which are "unlocked," i.e. you can overclock them freely. On a k-series Sandy Bridge or Ivy Bridge, you have a lot of freedom to change the CPU's "multiplier," which determines its maximum clock speed. For example, a multiplier of 37x on a typical i5 or i7 means a speed of 100MHz*37 = 3.7GHz. (The 100 MHz in that calculation is the BCLK, which you don't have much control over in sandy or ivy bridge.) But to overclock, people ramp that up to (e.g.) 45 or 47, so 4.5 or 4.7 GHz.
Even locked SB and IB processors can have a small overclock, though: you can set the multiplier to 4 steps higher than its maximum turbo frequency. So for example, on your 3450, the maximum turbo is 3.5 GHz = 35 multiplier. You are then able to set a multiplier of up to 39 = 3.9 GHz. That's not insignificant (more than 10%).