``time`` causes fish to measure how long a command takes and print the results afterwards. The command can be a simple fish command or a block. The results can not currently be redirected.
For checking timing after a command has completed, check :ref:`$CMD_DURATION <variables-special>`.
Your system most likely also has a `time` command. To use that use something like `command time`, as in `command time sleep 10`. Because it's not inside fish, it won't have access to fish functions and won't be able to time blocks and such.