The "IDs" (the number at the beginning of the line) are considered String IDs and are used by the Message_StringID functions in the source. The most common one we use is
Client::Message_StringID().
Basically, instead of the server sending the string, it sends just the string ID (so, less data over the wire). If the string ID has any variables (%1, %2, etc), you can also send a smaller string as part of the function that will be inserted where the variable is.
The idea behind EQ using a file like this is you don't have to recompile the EXE if you want to change what the strings output, especially when translating to another language.
As for the "cannot find string" error, I would recommend changing it in eqstr_en.txt also, although I'm pretty sure it's been depreciated (although maybe not entirely). It looks like the line endings are Windows-style & there is a new line at the end of the file, so as long as you maintain that, it should be fine.