LET'S START FROM THE BEGINNING! 

Generally, long before the appearance of the flag, many groups of people (kingdoms, empires, nations) used various SYMBOLS to differentiate themselves from other groups and to express their unity/homogeneity. You could find them on clothing, shields, documents, but not on flags!!!

War banners - which are ancestors of flags - have been widespread for centuries and were used as a means of communication *within* the battlefields.

However, the idea of attaching a piece of fabric vertically to a pole, instead of placing an object at the top (like a carved animal, for example), is owed to the Chinese, who were the first to use SILK!!

THANKS TO SILK, THE CONCEPT OF THE FLAG WAS "BORN" AS WE KNOW IT TODAY!!! 

The reason is that traditional fabrics were too heavy to be held up high and wave in the air - especially if they were painted - but silk was much lighter!!!

The new fabric, along with the new custom, spread over the years from the depths of Asia and reached Europe, which saw flags for the first time and copied them from the Arabs just 1000 years ago (during the Crusades)!

INTERLUDE: In case you don't remember what happened during the Crusades -> around 1000 AD, various Arab tribes had expanded throughout the entire southern Mediterranean and were advancing towards Europe at an alarming rate - thus began a period of about 200 years where Europeans and Arabs engaged in successive wars, pressuring each other to retreat.

So, flags arrived in Europe and became a trend around 1000 AD.

From the 1500s onwards, when "the age of discoveries" began (meaning the era when European peoples poured into the oceans in a competitive race to literally conquer the whole world), the concept of the flag was transmitted, adopted, or even imposed almost everywhere in the world!

In addition to nations, flags are now used as "visual slogans" by various entities: football teams, global organizations, companies, associations, etc.!!!