what does this symbol mean?(like a comparator,but there is another symbol in this triangle)

It is a comparator with hysteresis. Here is a diagram from this Maxim datasheet

When the output is in the high state the voltage required to drive it low (B) is higher than the voltage required to drive it high again (F), so for input voltages between F & B the output will be in either one state or the other, depending on the history of input voltage.

By incorporating hysteresis, a noisy signal can be "cleaned up" as the output will not transition many times for small changes in input voltage.

Schematic Symbol for a comparator with hysterisis or Schmitt trigger.

^{simulate this circuit – Schematic created using CircuitLab}

A shifted zero-crossing detector to deal with a noisy input.

Comparators have high gain, so a zero-crossing detector would transition between the rails for a signal oscillating near the reference.

You shift the reference point once it is crossed so the circuit is less susceptible to noise. Noise has to be greater that \$ V_{HYS} = 2 \times V_{UTP} \$ to cause a state change.

$$V_{UTP} = \frac {R_2} {R_1 + R_2} V_{CC} $$

Once Upper Trigger Point (Red) is reached, op-amp changes to \$V_{OUT}\$ shifting reference to Lower Trigger Point (Blue). Noise would have to be larger than Lower Trigger Point to transition.