The Replace Text option lets you change bits of a file name. You have two text boxes to fill in, with the Find box covering the text string you want to replace, and the Replace With box for what you want to put there instead. Here, it's used for the text string that is the filename. This option is quite straightforward, as it is similar to the 'Find and Replace' tools you may have used for text documents. There are broadly three categories of renaming you can use: Replace Text, Add Text, and Format.

Rather than explicitly naming each file, you're instead giving macOS a set of rules to use to rename them all. The Rename Finder Items pop-up gives you a few ways to rename the files you've selected. The trick is in deciding which of the possible settings you want to configure. On clicking Rename, all of the selected files will be renamed according to the settings within the Rename Finder Items dialog box.

Use the dialog box to rename your files.

With the files selected, either right-click the selected files and select Rename, or go to File then Rename in the Menu.

You can drag the cursor to select multiple with a box, or shift-click or Command-click the files, or in the Menu, select Edit then Select All.

Within a Finder window, select the files you want to rename.

How to batch-rename multiple files in Finder in macOS Dealing with multiple files isn't massively different than renaming a single file, to start with, but it quickly becomes a more complex task once you've started.

Macos batch rename