This may seem obvious, but I was completely unaware of the subtle difference until recently!
If you go to a website, you'll notice the address usually contains a http:// at the beginning. It stands for "HyperText Transfer Protocol" and is the standard way of transferring text to the internet.
If there is an "s" at the end of the abbreviation - https:// - that means that the site is "secure". A site becomes "secure" when the information on it will get encrypted (changed so that only the person with a key or password can see it) so that third parties can't view that information.
If you are entering personal or account information onto a website, make sure that little "s" is in the address!