This command blocks keyboard keys.  It can be used when it is necessary to disable keyboard input during macro execution, for example, before "wait for key" command (<waitfor>).  If it is required to disable keyboard keys during macro execution from the begin to the end, it is possible to use "Lock keyboard and mouse while macro is running" option in the macro settings tab.
To unblock keyboard, use <keys_unblock> command.

Syntax: 

<keys_block>
<keys_unblock>

Example: 

<#> This macro shows how to use 'keys_block' and 'keys_unblock' commands
<#>
<cmds>
<msg>(32,32,"Waiting untill 'a' key is pressed....","Message",0)
<keys_block>
<waitfor>("KEY","PRESS","a",50,0)
<keys_unblock>