Why does my PayPal say sandbox?

Good day, I'm encountering an issue with my PayPal account. I'm a bit confused as to why my PayPal interface suddenly displays "sandbox" when I attempt to make transactions or navigate through the system. Could you possibly clarify what the "sandbox" notation signifies and why it might be appearing on my account? Is it a security measure, a test mode, or perhaps an indication of some sort of error? I'd appreciate any insight you could provide to help me resolve this matter and ensure my PayPal account is functioning properly. Thank you in advance for your assistance.