In the Middle Ages it was really important when a knight was dressed in armour and out on the battlefield that he could easily be recognised and that he could tell who his friends were and who his enemies were.  That is why they had coats of arms which were very clear SYMBOLS of who they were.

Read about heralds and heraldry here. WHAT WAS HERALDRY? 

You can create a coat of arms on-line here then print it and read what your choices mean.  CREATE A COAT OF ARMS