Ki or, 気, is a very useful Japanese word. It has many meanings, but since there are many situations where this word is used, it would be beneficial for any Japanese language learner to get acquainted with it.
Ki 気 is the Japanese word for air; atmosphere; flavor; heart; mind; spirit; feelings; humor; an intention; mind; will. When used in conjunction with other verbs, this little word really starts becoming useful. I recommend studying this little word by looking it up in your Japanese dictionary, and noting some of its' uses. The following examples should get you started.
1. ki ni iru 気に入る- to take a liking to something
2. ki ga kawaru 気が変わる - to change your mind
3. ki ga sumu 気が済む - feel settled with something
4. ki ga nakunaru 気が無くなる - don't feel like it anymore
5. ki ni kakeru 気にかける- worried about something
6. ki ni naru 気になる- to be anxious about something
7. ki o tsukeru 気を付ける - to be careful with something