Putting is simply, is is when you access a website using HTTPS:// before the address in place of HTTP://.
Websites that use https:/// are using some cleaver encryption that ensures that information sent to the website browsers and from the website browser to the website is secured and cannot be intercepted by people spying on your connection.
Sites without this encryption send information back and forth as plain text. Making it super simple for anyone who is looking at this information to read all of it.
This includes passwords (and we all know we use the same passwords on different sites), to personal information such as when filling in a form. Email Address, Date of Birth, Address, Phone Numbers etc are just some of the information we want to ensure remains secure.