When we talk about synecdoche, we talk about a figure of speech in which one only uses a part of an expression but mean the whole expression.  It is a shortening which is understood because it is a common figure of speech.   For example, when we talk about society, we mean high society.
Dictionary.com’s definition of:
synecdoche 
noun
rhetoric
a figure of speech in which a part is used for the whole or the whole for the part, the special for the general or the general for the special, as in ten sail for ten ships pr a Croesus for a rich man.

The word comes from the Greek word meaning ‘shared understanding.’ It is a form of metonymy, wherein one word is used to replace another which is closely related to the original word.

synecdoche and scientific terms
“Within science writing, synecdoches are common as well. For example, DNA is a synecdoche for life, the test tube for an experiment, the statistical test for proof, and Tally’s corner for a kind of social organization.”  Laurel Richardson –  Writing Strategies

Put simply, synecdoche is when we use a word or phrase to represent something else.

everyday examples of synecdoche

The term ‘plastic’ for a credit card
The word ‘bread’ can be used to represent food in general or money  he is the breadwinner; music is my bread and butter).
The word ‘ivories’ refers to piano keys ( which in the past were made of ivory)
A golfer’s ‘woods’ mean their longest golf clubs.
Facial tissue is often referred to as ‘Kleenex’s
Vacuum cleaners are called Hoovers

It is a common part of speech, and now you know what it is you will recognize a synecdoche when you see  it. It is a great literary tool if you want to use visual imagery, and appears a lot in poetry and song lyrics.