Imagery is when language forces someone to create a mental picture of something.  It is all about painting a picture with words. For example:
“A wine shop was open and I went in for some coffee. It smelled of early morning, of swept dust, spoons in coffee-glasses and the wet circles left by wine glasses.” ― Ernest Hemingway, A Farewell to Arms

In literature, one of the strongest devices is imagery.  The author uses words and phrases to create “mental images” for the reader. Imagery helps the reader to visualize the writer’s meaning.

“The usage of metaphors, allusions, descriptive words and similes amongst other literary forms in order to “tickle” and awaken the readers’ sensory perceptions is referred to as imagery. Imagery is not limited to only visual sensations, but also refers to igniting kinesthetic, olfactory, tactile, gustatory, thermal and auditory sensations as well.” – Literarydevices.com

examples of imagery

Here is an example of imagery from classical literature:
“The world is a book and those who do not travel read only one page.” ― Augustine of Hippo

Songs from the dawn of time up until today use imagery in abundance.  “Lucy in the Sky with Diamonds” and many other Beatles songs use very vivid imagery.
Imagery  is one of the main – if not the main – vehicle used by poets and lyricists (song-writers) to convey meanings, feelings and experiences to their readers and listeners. Imagery is what poems are about, and this applied to songs, which are really poetry set to music. Many would state that the most beautiful way to describe and convey feelings  is to use imagery.