Understanding contents and workwise of a Terraform plan
Terraform use protobuf to store the plans according to its code


  // writeTfplan serializes the given plan into the protobuf-based format used
  // for the "tfplan" portion of a plan file.


You can find here the code generating the plan file.

So in brief that's a binary file with a serialized version of what to do in a portable format (protocol buffers) for which you find more information here