First off, a macro is a series of commands or instructions stored in an Excel module. Macros can be run anytime you need to do tasks related to those instructions.
If you know how to make a macro in Excel, you would be able to save so much time and even minimize human errors.
The problem is, macros can be used for good and for evil. Macros can literally be used to delete files on your hard drive.
For that purpose, Excel, by default, will not run a macro-enabled worksheet.
If you trust the macro and you would like to use it, then you would have to manually enable it. You can even set up your Excel to always run macros for a trusted worksheet.