The https is encryption you get when your website is using a certified SSL. SSL stands for Secure Sockets LayerLayers of safe harbor in Castilian ... although almost certainly not this helps you understand that do exactly the SSL certificate and https.
In plain words, use an SSL certificate and therefore data encryption your website gets the following:
1) encrypts data so that if something were in the middle of that data stream and tried to read, I could not do it. You could see the data packet, yes, but not understand them because they are encrypted.
2) Ensures data integrity while being transferred because they can not be altered or intentionally or by mistake.
3) Provides authentication communicating to users that your site is really you say it is so the user can be sure that's your website and not a copy created by someone to steal the access data to your account whatever it will be stolen.
It is especially important in places where there are user registration and even more important in e-commerce sites.
Does https improves SEO?
If you want to read the official statement from Google, you can do so here: "Https as ranking signal”