The purpose of an idiom is to express the meaning in a subtle, indirect manner, thereby making it unique and attractive. It sets the reader's mind to think beyond what is written, to understand the hidden meaning, thus making the reading more interesting.
The following are some common idioms, with meanings.
  1. Cool as a cucumber - calm, not nervous.
  2. Under the weather - not feeling well.
  3. To give up - admit defeat.
  4. Over the moon - extremely happy.
  5. Once in a blue moon - very rarely/almost never.
  6. Piece of cake - too simple to accomplish.
  7. Blessing in disguise - an unexpected good turn of events.
  8. Have hands full - very busy.
  9. Minting money - making quick and easy money.
  10. Took to heels - ran away.
  11. In the nick of time  - just at the right moment.
  12. Make up my mind - decide.
  13. Turn over a new leaf - reform or change for better.
  14. Take to task - punish.
  15. In the long run - over a long period of time.
  16. Smell a rat - Have a reason to suspect something wrong.
  17. For good - permanently.
  18. Go  out of your way - take extra trouble.
  19. Leave no stone unturned - make use of every available opportunity.
  20. Blow your own trumpet - praise yourself.
  21. Caught red-handed - Catch someone in the moment of theft.
  22. Greek and Latin - unable to understand.
  23. Mince words - speak vaguely.
  24. Wild goose chase -  fruitless search.
  25. Wolf in sheep's clothing - a person who pretends to be good.
Study more such idioms here.