A plugin is a tool which can be link to a QPU to make the computation stack more powerful. A plugin is used to pre-process batches sended to a QPU and/or post-process results of a QPU. When a Plugin is linked to a QPU, a new QPU is created. For instance, the following code defines a new QPU where each batch sended to this object is pre-processed and each result is post-processed by the plugin "Plugin()":
my_qpu = Plugin() | QPU()
The QLM provides classes to define your own plugin, an example of custom plugin is presented in this tutorial.
Moreover, some plugins are already implemented in the QLM:
Finally, this notebook shows how to write your own QLM compatible plugin.