In this case, "en-" acts as an intensifier, indicating the process of "surrounding" or "going around" something. It's combined with the word "circle" to create the verb "encircle," which means "to surround completely".
Here are some other examples of words using the prefix "en-":
* enclose: To surround or confine
* engage: To become involved or participate
* endure: To withstand or tolerate
* enlarge: To make bigger
While "en-" doesn't have an independent meaning, it helps to create a new meaning by modifying the base word it's attached to. In the case of "encircle," it clarifies the action of "surrounding" something completely.