what tier data= mean?

tier data=%meta_desc%  how to understand this?

I know it will extract meta descrtion from current page, but where to input this?

This will exchange your defined content in the project with the one from that variable if the content looks good (e.g. long enough, not too short,...).

that is my problem. where is my defined content?
where is tier data?

If you have ...

[Description_250]

...

tier data=%meta_desc%

...

You have the default content in the project file.