Your Apple ID was used to sign in to iCloud via a web browser.

If you actually signed in with a web browser then there's nothing to worry about or do. The change password suggestion is if you did not sign in and somebody else may have discovered your passwrod and is signing in as you. In that case here's several articles covering compromised IDs:

Click on this link "If you think your Apple ID has been compromised" --> If you think your Apple ID has been compromised - Apple Support

For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> See who has access to your iPhone or iPad - Apple Support

For iOS 16 or later see how Safety Check on iPhone works to keep you safe. How Safety Check on iPhone works to keep you safe - Apple Support

Related materials:

Personal Safety User Guide

Personal Safety User Guide - Apple Support

> open the Table of Contents

A document with general information about security and your Apple ID --> Security and your Apple ID - Apple Support

Contact Apple for help with Apple ID account security by clicking on this link --> Contact Apple for help with Apple ID account security - Apple Support "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."