I got a little distracted when I started thinking about answering today's question. A few years ago I was speaking at a university in the midwest and I stopped at a Kum & Go convenience store. I had never heard of the store and couldn't believe that people didn't think of the name in sexual terms. Even when I asked local college students about it, they said they didn't think of it as sexual. However, I wasn't alone in my interest in the name since I later learned that Johnny Knoxville wore a Kum & Go t-shirt in Jackass Number Two movie. But I'm getting a little sidetracked so let's get back to the question:
Yes, "cumming" is a slang term that refers to orgasm, but usually only the male orgasm (the slang word "cum" refers to semen).
Talking about sex can be difficult and what makes it even harder is trying to figure out what type of language to use. Some people like to use clinical or scientific language (terms such as "orgasm," "coitus," or "ejaculation"), while others prefer to use sexual slang (using terms such as "making love," "hooking up," or "busting a nut"). Although clinical language can seem sterile and cold, sexual slang can often be confusing. Like you, many people are unsure of the meaning of many slang terms, so it's possible to be confused if your partner uses it.
At the end of the day, however, it's up to you and your partner to decide what works best. We know that couples who talk about their sex are happier and more sexually satisfied than those who don't talk about it!