By order in this TV series means: This is an order

The entire line is actually "By order of the Peaky Blinders", which means "This is an order from the Razor Party."

In conjunction with the TV series, this line can also be translated in multiple ways as follows:

This is the order of the Razor Party;

This is the execution of the order of the Razor Party;

The Razor Party asked me to do this;

As for why this line is repeated, it is actually commonly used in foreign film and television dramas. For example, the same line is also used extensively in "Game of Thrones" Lines, for example:

"Yes, my lord (obey, my master)"

"winter is coming"

If a line is repeated a lot, the subconscious will label the entire play.

This is actually the same as when we watch advertisements every day. Just like when you see a line, you will think of the corresponding advertisement. When you see the advertisement, you will guess the next line. For example, when you say urgent syrup, you will think of it. Cheetah.

Similarly, when you hear BY ORDER, you will think of Peaky Blinders!

In order to facilitate understanding, you can also think like this.

Order means "list, menu".

It means (you) are in the menu of the Razor Party, which by extension means that you will be ordered sooner or later!