Man-in-the-middle or MITM attacks are a very simple kind of cyber attack: A hacker gets in the middle of your connection with someone else.
Let’s say I’m trying to access my email, but some hacker man-in-the-middle’d my connection to my email provider. So my connection would look like this:
I’d still be talking to my email provider, but the hacker would be able to read and save everything. If I type in my username and password, he’d be able to read those too. Not just that, he could change the messages that my email provider and I would be sending to each other, and block messages that he doesn’t want received. Worst of all, he could connect me to whatever IP address he chooses; instead of sending me to Gmail when I type in gmail.com, he could send me to a completely different website that puts malware on my device.
This used to be a very common problem that was difficult to avoid, but HTTPS has solved this issue.