The Attack control sets the attack time, or the rate at which gain is reduced after the input signal crosses the threshold.
The smaller the value, the faster the attack. The faster the attack, the more rapidly the Compressor/Limiter applies attenuation to the signal. If you use fast attack times, you should generally use a proportionally longer release time, particularly with material that contains many peaks in close proximity.
This control ranges from 10 μs (fastest attack time) to 300 ms (slowest attack time).