It's from Shakespeare's Henry V and it's part of the speech given by the titular King to his troops just before the Battle of Agincourt - which fell on St. Crispin's day. The Battle of Agincourt became a major English victory against the French during the Hundred-Year War and led to Henry V being recognized as Heir Apparent to the French crown, and his son, Henry VI, claiming the title of King of France as well being King of England.
In simpler terms the quote is saying that the glory gained by fighting in the battle alongside their King will be so great that any man who could fight who was not there will be jealous and will think of themselves as lesser men when in the company of those who fought.
The whole speech is here - https://en.wikipedia.org/wiki/St._Crispin%27s_Day_Speech - and well worth reading.
Is that of any help?
(some context) This was part of a motivational speech by Henry V, given before the Battle of Agincourt, which took place on St Crispins Day.
The notion isn't new, Shakespeare as usual sums it up:
And gentlemen in England now-a-bed Shall think themselves accurs’d they were not here, And hold their manhoods cheap whiles any speaks That fought with us upon Saint Crispin’s day.