robots communicating with each other
You are basically asking how two objects/circuits can communicate with each other. There are of course many ways this can be accomplished:


Wired communication: SPI, I2C, UART, RS-232, etc.
Wireless communication: 2.4Ghz, Sub-Ghz, Infrared transmission, etc.


Clarifying what exactly are the application of the robots, the environment, the desired distance between them, etc. could help decide on a specific answer/recommendation.