What does it mean to collate in a printing or copying context?

In terms of printing or copying, collating refers to putting printed sheets or photocopies in proper order, especially for binding. When you're printing or photocopying something and you're only printing or making one copy, you don't need to address the idea of collation at all. But if you're printing or making multiple copies of multi-page documents, you might want the printer or photocopier to collate the pages.

If you select the "collate" option, the documents that emerge all nicely printed or copied will have their pages in order already; you'll have a stack of documents all ready to go. If you choose not to collate the documents, you'll have a stack with all the first pages together, all the second pages together, all the third pages together, etc., and you'll have to assemble each document by hand. Collating, then, can save a lot of time.