Effect of zeroes in stability
Consider a standard inverting amplifier built around an op amp with a standard two resistor feedback network. Now we place a small capacitor (pF) across the feedback resistor. If the capacitor value is carefully chosen stability can be improved. The added capacitor has added a zero in the loop transfer function. That is to say as frequency increases, the loop gain is increased above the zero frequency at a rate of 20dB/decade which actually reduces stability but a greater effect is caused by the added 90 degrees of phase lead, added to the loop phase, improving stability.

Note the frequency of this zero must be chosen carefully because the added capacitor also adds a loop pole at a higher frequency than the zero which can have a destabilising effect.

There can be a confusing element to the added capacitor adding a zero in the loop. That is that the added zero, when viewed from the point of view of the loop, can be viewed as a pole when viewed from the point of view of the closed loop response. That is to say it roles off the closed loop gain and therefore is actually a pole in the closed loop response. 

It is the rate of conversion of the open loop response and the 1/B curves which determines stability. The zero reduces the rate of convergence between these two responses improving stability so long as the 1/B curve crosses the open loop curve before that higher frequency feedback pole comes in.