CIM stands for cum in mouth, where you ejaculate into your partner's mouth. It is just one of the many ways in which you can end your session, if your partner is happy to try it. It can be a fantastic way to end, as your partner may wish to finish you off with their mouth, meaning that moving to a new position isn't needed. You can simply enjoy the sensations being given to you by your partner and let things happen more naturally.
Finishing in this way might be fun for you, but you should at least discuss the idea with your partner first. They may not feel comfortable doing it for many reasons. For some, the taste of cum is too much and they cannot stand it. For others, the idea of cum being in their mouth just grosses them out. While you can try changing your diet to improve the taste (healthy eating is preferable here), your partner still may not be open to the idea. If they aren't, respect their wishes. There are plenty of other things you can do instead!
This might be a no brainer, but when it comes the time to release, give your partner some warning. We're not suggesting screaming it from the rooftops, but a simple "I'm cumming" will suffice. They can prepare for your load better that way, and are less likely to jump at the sudden surprise you give them.
Sometimes that many people worry about is the old "to swallow or to spit" dilemma. You may not have a preference, but a good number of people do, and your partner might be one of them. If they choose to spit, try not to take it hard. It doesn't mean they find you disgusting - it could just be that they are worrying about their health. Talk to your partner afterwards and discuss what you would do differently next time. It may be that they wish to try swallowing, or that they've like to finish you in this way while you perform reverse oral on them.