for example, let’s try this:

mkdir {a,b}/{1,2,3}

this will create the following structure:

a/1
a/2
a/3
b/1
b/2
b/3

The posibilities for this are really interesting, and it is a powerful feature. For example, let’s say that you have files for 02-28 and 02-27 and want to move them:

# ls -1
log_02-28
log_02-27
# mv (or cat, or cp, or whatever) log_02-2{7,8}

Bash is powerful, but you need a lot of practice to master it. I’m proud to be slowly walking that road