There is no option to set the field hidden in the filed level security. You mean you are removing read/edit access to that field, if so you can access that field in Apex code, but while running it will throw error message to the user if user don't have access read/write access to that field.
Note: I strongly recommend you to do some research and post your question in forum. You are asking question which can be easily find in google and salesforce documentation and moreover its looks like an interview questions.
There is no option to set the field hidden in the filed level security. You mean you are removing read/edit access to that field, if so you can access that field in Apex code, but while running it will throw error message to the user if user don't have access read/write access to that field.
Note: I strongly recommend you to do some research and post your question in forum. You are asking question which can be easily find in google and salesforce documentation and moreover its looks like an interview questions.
To get start with development check this Trailhead modules which will be helpful for you to learn - https://developer.salesforce.com/trailhead
Salesforce Documentation - https://developer.salesforce.com/docs
Workbooks Tutorials - https://developer.salesforce.com/page/Force.com_workbook
Thanks,
Karanraj