Answer:
Oscillations are periodic motions of some objects.
For example, the swing back and forward of a telephone pole when the wind is strong is an oscillating type of movement, which depending on the object (mass, shape, volume) it has a natural frequency in which oscillates more easily.
This is the same for the tuning fork when you hit it against something, you give the tuning fork some energy that it "loses" by the vibrating motion. And it "vibrates" with the natural a frequency that is equal to the note that the musician needs to tune in his instrument.