From time to time, Apple changes its agreements and when they do, Apple requires the person who is the "Account Holder" on the account to login and accept the latest agreement.

To accept the latest agreement;

Sign in to https://developer.apple.com/account and click on the "Review Agreement" button in the top, colored banner. 

or

 



 
Right-click and select "Open in new tab" to enlarge

 Login to https://appstoreconnect.apple.com and select "Agreements, Tax and Banking". 

The status column on the "Agreements" page will indicate what action is needed.