This is a configuration file which is read by u-boot prior to handing off to the kernel.
The “LINUX” line is the path to the kernel, e.g.:
The FDT line is the path to the binary data tree blob for firmware, usually named for the board surrounding the SoC, e.g. the PM375 of the TK1:
The line most people will be interested in is the “APPEND” line, which is kernel command line argument (it’s possible other parameters were coded elsewhere, hence this “appends” and doesn’t “replace” the kernel command line). To know what the content of this line is you’d need to know what arguments that kernel accepts (which in turn depends on kernel version, what features are compiled in, and what features are loaded as a module). Note that this huge list of kernel arguments is still a single line…your editor may line wrap, but truly multiple lines would be incorrect.
Other lines are basically for selection of which LABEL to boot. Default L4T has only one label, but others can be added and interactively chosen via serial console during early boot.