In a nutshell, shocking means to raise the chlorine level high enough to cause a reaction that burns up bad stuff. The chlorine level needed to do this depends on your CYA level. Measuring CC (combined chlorine) is the way to determine of you have enough bad stuff in your pool to shock. Generally 0.5 or under is OK for CC, above that you can shock to bring CC down to 0.

Having a lot of people in the pool can cause CC to go up so it's a good idea to check after you have a lot of people in the pool. An FAS-DPD test is what you need to accurately measure CC. The OTO kit that tests chlorine and pH tests for total chlorine which is CC + FC (free chlorine) so there is no way to tell from the OTO result how much FC vs CC you have.

Peter