An event parameter lets you capture information about an event. It's written as key-value pairs, where the value on the left is the event parameter name and the value of the right is the event parameter value.
You can analyze the data from event parameters using dimensions and metrics. Anytime Google doesn't provide a dimension or metric for an event parameter, you must create a custom dimension or metric in order to see the event parameter value.
To learn how to set up an event parameter, see Set up event parameters.