As for the actual question in the title, the command means
./cmd/tree-tagger-english
run this command ...
<
... with standard input from ...
$f
... an (incorrectly unquoted) variable which should contain a file name ...
>
... with standard output redirected to ...
$f.tagged
... a file named by the (still incorrectly unquoted) value of $f
with the string .tagged
appended at the end ...
&
... as a background job.
Apparently ./cmd/tree-tagger-english
in turn attempts to execute a command which doesn't exist, many times.
Probably the instructions tell you to run the thing in a different directory than where you are actually running this; but this is obviously merely a speculation. Another fairly common scenario is that something failed during installation, but you failed to notice (and so some files which should exist in these locations do not actually exist ... disk full? Wrong permissions?)
The fact that the code contains quoting bugs suggests that it's also quite possible that the code is simply buggy.