在一些 vim 映射后面的 < CR > 是什么意思？

It's basically a means to say "this is the end", see map:

When you have a mapping that contains an Ex command, you need to put a line terminator after it to have it executed. The use of is recommended for this. Example:

:map _ls :!ls -l %<CR>:echo "the end"<CR>

also,

<CR>    [count] lines downward, on the first non-blank character |linewise|.

<CR> in a mapping corresponds to the Enter key just like a in a mapping corresponds to the A key. Ley's say you have this mapping

:map <f8> :wq<cr>

This will map F8 to the key sequence :WQEnter (which would save the current buffer and quit).

Why <special keys>?

While you can use literal keys in mapping definitions (the Enter key would appear as ^M, or even just as an additional new line, depending on the settings), Vim provides a special key notation for key (combinations), so that it is easier to define (you don't have to use i_CTRL-V to literally insert the special character) and understand (<A-a> better expresses the intention than the equivalent á) the mappings.

See :help key-notation for a list and explanation.

Why <CR>?

As many mappings invoke Ex commands (e.g. :w) and therefore have to switch from normal to command-line mode, they have to conclude the command with <Enter> (or <CR>), just as you would when manually typing the command.

:help key-notation

says:

notation    meaning            equivalent  decimal    value(s)
-----------------------------------------------------------------------
<CR>        carriage return        CTRL-M    13       *carriage-return*
<Return>    same as <CR>                              *<Return>*
<Enter>     same as <CR>                              *<Enter>*

Mappings often involve Ex commands and you must press <CR> to execute them so it's included in the mapping.

The <CR> in vim mappings is the carriage return usually the Enter on your keyboard.