The click-to-open rate (CTOR) compares the number of unique clicks to unique opens. This number indicates how effective the email message, design, and content performed, and whether it created enough interest in the recipient to take action.
Unlike click-through rate, click-to-open rate is the number of clicks out of the number of opens (instead of the number of delivered emails). This gives you a better gauge of how the design and messaging resonated with your audience, since these clicks are only from people that actually viewed your email.