What is "tCAN" in codes of CAN bus?
tCAN will be a struct that defines the structure of the CAN message being sent.  The name tCAN is not a standard and will have been chosen by the author of the library you are using.  At a guess it's using a variant of Hungarian Notation, and the t prefix means type - so it's probably been typedef'd from a struct to a type name.

I am not familiar with CAN, but if there are standard message formats this struct will correspond with one of them.  You can find the actual structure within the header file for your chosen library (or maybe in the example sketch code itself?).