A Mapping ID is the unique 'link' between a row of data within your spreadsheet and the associated activity in which you would like for the data to be housed in within your account. This column is only required when you wish to use the 'Multi-Site Upload' option in order to upload data from a spreadsheet to multiple activities at once. 

Mapping ID's are used to ensure data is uploaded from the Data Upload Template to the correct activity in the system. Mapping IDs for activities and metrics can be found at the locations shown in the image below: