I can't understand how unsigned char _ [32] is used in this example

I am working on some examples of using zeromq and started reading the zeromq.h file to get into more detail.

I am buffled at the line where the zmq_msg_t is defined:

https://travlr.github.io/libzmq/zmq_8h_source.html#l00129

typedef struct {unsigned char _ [32];} zmq_msg_t;

For example you can initialize a buffer like this:

zmq_msg_t buffer = {0};

My question is probably about the compiler stuff I don't quite understand. Why do you define a structure like so:

typedef struct {unsigned char _ [32];} zmq_msg_t;

Is this a unsigned character array of 32 characters? What is the use of "_" ?

Thank you in advance.