What does it mean Imagery ?

Imagery means the use of metaphorical mental imagery to support the development of healthy physical function, strength, mobility, and coordination.

In other words, it’s the combination of visualization techniques with physical exercises to improve movement.
Imagery techniques help to improve coordination, body awareness, flexibility, mobility, strength, and understanding of the body.

Imagery work is wieldy used in dance, yoga, Pilates, physiotherapy, and many other disciplines.

Pioneer in this field was Mabel Todd that conceived the Ideokinesis approach:  https://en.wikipedia.org/wiki/Ideokinesis
http://ideokinesis.com/ 
Another approach that nowadays it’s the Franklin Method 
https://franklinmethod.com/