The `time` you use in your question is not a command, but rather a _reserved word_ in [tag:zsh]. It can be used in two different ways and its output can be customized through the `$TIMEFMT` parameter. Additionally, there is an _external command_ `time` on macOS (and most other Unix-like OSes), which produces slightly different output.

## Just `time`

If you use `time` by itself (as shown in your question), then your current shell will report time statistics for itself and its children:
* `shell` represents the shell in which you executed your `time` statement. `children` is a summary of all running processes started from (and still owned by) that shell.
* `user` and `system` show the amount of CPU seconds the above processes spent in [user mode and kernel mode](https://blog.codinghorror.com/understanding-user-and-kernel-mode/), respectively.
* `cpu` shows the combined values of `user` and `system`, as a percentage of total CPU time. `total` shows how long ago the shell or its oldest child process was started (here in minutes:seconds.fraction).


## `time <command>`
More often, `time` is added before a command. If you use it that way, then the shell will report time statistics for that command only:
```zsh
% time sleep 0
sleep 0  0.00s user 0.00s system 43% cpu 0.007 total
% 
```
`total` here is the number of seconds it took for the command to complete.

## Customizing `time` output

`time`'s output can be customized by setting the [`$TIMEFMT` parameter](https://zsh.sourceforge.io/Doc/Release/Parameters.html#index-TIMEFMT). For example, we can add memory usage:
```zsh
% TIMEFMT+='  max RSS %M'
% time        
shell  0.22s user 0.10s system 53% cpu 0.601 total  max RSS 6120
children  0.11s user 0.16s system 45% cpu 0.601 total  max RSS 3056
% time sleep 0
sleep 0  0.00s user 0.00s system 61% cpu 0.003 total  max RSS 580
% 
```

## External command `time`
The external command `time` can be used as follows:
```zsh
% command time
% command time sleep 0 
        0.00 real         0.00 user         0.00 sys
% command time -p sleep 0 
real         0.00
user         0.00
sys          0.00
% command time -l sleep 0  # Show memory usage
        0.00 real         0.00 user         0.00 sys
              585728  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
                 160  page reclaims
                   0  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                   0  voluntary context switches
                   1  involuntary context switches
             2198249  instructions retired
             3195521  cycles elapsed
              237568  peak memory footprint
%
```

`real` here is the same as `total`, above.