Most work stuff I do is on a branch named with a JIRA issue key (and all commits prefixed with the key as well), so I can go read the JIRA if the commit messages aren't enough context.
If the changes are uncommitted there's a good chance they'll at least be on branch abc-123 If there are commits the the log is likely to say something like ABC-123 Progress commit with rough implemention of the new turboencabulator.
But realistically if the team had made lots of progress and even I couldn't remember what the changes were, I'd probably hard reset and move on. Not all code is worth keeping, particularly not code written just before going on a break ;)
If you're concerned it may actually be important and you'll remember in time, stick the changes in a branch like 20180609-mystery-changes and store them in a fork. Git branches are cheap after all.
If you really really have to work out what it was; you're going to have to check the log to see if a previous commit message jogs your memory; and look at the diff and just straight-up figure it out from first principles.