Debug versions of i3 automatically use 1% of your RAM (but 25 MiB max) to store full debug log output. This is extremely helpful for bugreports and figuring out what is going on, without permanently logging to a file.
With i3-dump-log, you can dump the SHM log to stdout.
The -f flag works like tail -f, i.e. the process does not terminate after dumping the log, but prints new lines as they appear.