OneDrive is an online (cloud) storage area for you to store/backup your files, it is fully integrated into Windows 11 so you can easily access your OneDrive files using File Explorer, or via the OneDrive website from any computer.
If using a Microsoft Account by default all your documents, and photos from your camera will automatically be stored on OneDrive, as well as all your Windows settings so that you can have the same settings across all of your devices (i.e. start menu layout and appearance, desktop personalization, app settings, internet history and favorites, language/keyboard/mouse settings etc)
If you use Microsoft Office then this also fully integrates with OneDrive so you can save all your files in the cloud automatically.
When you first create a Microsoft Account you automatically get 5Gb of free OneDrive storage and can upgrade to more space (up to 100Gb) if required for a small fee each month, if you have Microsoft 365 (formerly Office 365) then that includes 1Tb of storage per user (1 user with Microsoft 365 Personal / 6 users with Microsoft 365 Family)
If you are using a Microsoft Account to sign-in to Windows 10 then you will automatically see your OneDrive account in File Explorer (as highlighted below)
From here you can access any files stored online, create new files/folders, delete any files you don't want etc.
You can also right mouse click on a file or folder and choose to make it available offline - handy if you are going to be using your computer without an internet connection for a while, once you are reconnected to the internet then any changes to the file/folder will automatically be backed up.
You can also access all files stored in your OneDrive from any computer by visiting www.onedrive.com