Backseat driver

Meaning:
A backseat driver is a person who constantly tries to correct another person while they are trying to complete a task. A backseat driver is seen as an annoyance and his/her suggestions are unwelcome.
Example Sentence:
My mother in law is such a backseat driver, always trying to correct everything I do.

Back to the drawing board

Meaning:
To go back to the drawing board means to start over on a project. For example, you could complete a project for a boss but your boss decides he or she does not like the results and you have to go back to the drawing board.
Example Sentence:
The car designer decided he did not like the finished look of the car after all so he decided to go back to the drawing board.

Cash in your chips

Meaning:
To cash in your chips means to sell something before the value of it drops. Alternatively, it can also mean to die.
Example Sentence:
Example 1: He could see that the stock prices were going to fall, so he decided to cash in his chips.

Example 2: When the plane crashed he cashed in his chips.

Elephant in the room

Meaning:
A problem that is impossible to overlook so everyone knows about it yet no one wants to discuss it.
Example Sentence:
I always recommend to address whatever happens to be the elephant in the room as soon as possible so hopefully everyone will feel more relaxed once the issue is out in the open.

Fall by the wayside

Meaning:
When something falls by the wayside, usually because people lose interest or become preoccupied with something else.
Example Sentence:
So many projects seem to fall by the wayside because we always start new one’s before we complete the old one’s.

Batten down the hatches

Meaning:
To batten down the hatches means to prepare for a bad event. This idiom comes from when sailors would prepare for a storm by covering their hatches.
Example Sentence:
The new the hurricane was coming but instead of evacuating they decided to batten down the hatches and stay.

A blessing in disguise

Meaning:
Something that initially seems negative or like bad luck but turns out to be a good or positive experience in the long term.
Example Sentence:
Getting fired from my last job was a blessing in disguise that allowed me to find my true passion and begin a much more fulfilling career.