[stomachmonkey]

https indicates a secure encrypted connection.

there are a variety of reasons that sites will throw a certificate error.

it can be as simple as the certificate being set up as "somesite.com" but the url it takes you to is "www.somesite.com" where the "www" is the mismatch.

the cert could also be expired.

another one you see a lot is "contains insecure....."

generally when someone is too lazy to host images in the https directory and links to them from the http directory or if they throw their insecure google analytics cookie on the secure page because they want to track cart conversions.