Halfway through, but not yet | "in progress of doing something".
- 昨日買ってきたイチゴが、もう腐りかけている。The strawberries I bought yesterday have begun to rot.
- 食べかけのケーキにラップをして冷蔵庫に入れた。I wrapped the half-eaten cake and put it in the refrigerator.
- 家の前で死にかけていた野良猫を拾って動物病院に連れていった。I picked up a dying stray cat in front of my house and took it to a veterinary clinic.
It can mean that the specific action is half done without ending (such as the second example sentence); it can also mean that a certain state has begun to occur (such as the first and third example sentences).