Dynamics Plugins come in 4 different varieties; compressors, gates, limiters and expanders. As the name suggests, Dynamics Plugins are used to modify the dynamics of an audio signal.
Compressors are used to level out the louder and quieter parts of an audio signal. This creates a more unison sound and allows for boosting of the overall volume level of the audio signal.
A gate is used to remove sound below a certain threshold. Until the threshold is reached, no audio signal is passed. This type of plugin is for example used to remove noise in audio signals.
A limiter is used to prevent the audio signal to go beyond a set threshold. A limiter is often used to increase the overall loudness of an audio signal.
An expander is used to increase the dynamic range of an audio signal and does the opposite of a compressor. This type of plugin is used to make the audio signal that passes a set threshold louder, rather than softer as a compressor would do.