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It's a hex dump, in a fairly common hexdump format, with each line of output containing the byte offset (in hexadecimal), hex digits for 16 bytes, and the ASCII representation of those same bytes.

From man xxd:

NAME

   xxd - make a hexdump or do the reverse.

SYNOPSIS

   xxd -h[elp]
   xxd [options] [infile [outfile]]
   xxd -r[evert] [options] [infile [outfile]]

DESCRIPTION

xxd creates a hex dump of a given file or standard input. It can also convert a hex dump back to its original binary form. Like uuencode(1) and uudecode(1) it allows the transmission of binary data in a 'mail-safe' ASCII representation, but has the advantage of decoding to standard output. Moreover, it can be used to perform binary file patching. [...]

hex dumps are extremely useful when you need to know exactly which bytes are in a file at some exact location. They can help to diagnose problems where, for example, there are unexpected bytes (e.g. NULs (0x00) or carriage-returns (0x0D)) in the file.