I have a checkbox in my vf page which is using custom controller. I need to get check box value true in apex class controller when it is checked, so that I can perform my logic based on that checkbox value. How to do that?

After clicking the check challenge button, it throws error but it didn't show what the error is?

I have a checkbox in my vf page which is using custom controller. I need to get check box value true in apex class controller when it is checked, so that I can perform my logic based on that checkbox value. How to do that?