What is meant by information in special theory of relativity

What is information?

Is it a thing or a concept?

If it is a thing how small can it be?

Information is anything that conveys knowledge of or about something. I suppose the simplest method currently used is the binary system, which is just a string of 1's and 0's, but look what can be done with those!!
This is what Einstein meant by information. It is impossible to send knowledge of or about something faster than the speed of light.

Special Relativity does not address information, just the max. speed of propagation of any signal (event).

For the rest, use Google.