Wildcards match names that exist. To make new files or directories, you need a way to write many names quickly. That is brace expansion.

One word for each item

Put items between { and }, with commas between them. The shell makes one word for each item, with the text around the braces:

learner@tidepool:~$ echo {mon,tue,wed}.txt
mon.txt tue.txt wed.txt
learner@tidepool:~$ touch days/{mon,tue,wed}.txt

No spaces go inside the braces. {mon, tue} is not brace expansion.

Make a backup copy quickly

An empty item gives the text around the braces with nothing added:

learner@tidepool:~/shop$ echo prices.txt{,.bak}
prices.txt prices.txt.bak
learner@tidepool:~/shop$ cp prices.txt{,.bak}

Braces or wildcards?