What is the difference between "100 monthly submissions" and "10 secure submissions"?

The difference between the 2 is that for the regular submission you are using HTTP protocol which means that there is no encryption protecting the data entered on the jotform.

Now, with the use of secure submissions, it means that your forms are opened over HTTPS protocol, which means that not even your ISP knows what you are seeing nor what you are entering - in this case, the same applies to your visitors - anyone that comes to the jotform.

Both of these are based on monthly limit, so they would all reset to 0 on 1st of each month.

Please do note that you can either use 10 Secure submissions, or 10 paid submissions or 100 regular submissions. Once any of the type is used up, your account is disabled unless upgraded - that is until the start of next month.