Let's say you want to test the login for an application. Here's how you'd go about storing username and password in Testery.
Navigate to Environments Tab
Add a new environment (if you don't have one)
Add a variable for your sensitive data like password
Enter name of variable (i.e. password) as CYPRESS_password and its value. For Cypress, you'll be able to access the value using Cypress.env('password') in your code (omitting the CYPRESS_ prefix)
Click on "encrypt" checkbox (screenshot below)