It indicates the action of closing off, shutting in, or bringing together. This can be seen in words like:
* Include: To bring something in; to make it part of a group.
* Exclude: To keep something out; to prevent something from being part of a group.
* Conclude: To bring something to an end; to decide or determine something.
* Seclude: To isolate or separate something; to keep it away from others.
Essentially, the suffix "-clude" implies a sense of limitation, restriction, or containment.