Hidden fields define the value that is never displayed to the user. The user cannot interact or change the value of the field. 

It is particularly useful when you want to add a tag or note to all contacts that fill up a particular form without them knowing. 

To use hidden fields, 

i. Navigate to the Marketing dashboard. 
ii. Go to Forms > Inline Forms. 
iii. In the upper right corner, click Create Inline Form
iv. Start by adding a name to your form. Once done, you can either choose from a predefined style for your form or create a custom one. 
v. Under the Field tab, click on Add New Field. From its drop-down menu, select Hidden Fields
vi. Enter the detail and choose if you want to Add as Tag or Note. 
vii. Hit Save

Whenever a contact fills up that form, the assigned tag/note will be added to the contact’s record. You can use that tag to create a smart list of contacts for running targeted campaigns.