Don't confuse this with people who don't want you to know what they're buying. Those are "consumers with discretion."
"Consumer discretionary" refers to a portion of consumer spending that is used on things that people don't really have to buy. The goods and services are not essential, at least not in a live-or-die kind of way. You can buy or not buy, based on your discretion. It's optional. Up to you.
The concept comes opposed to non-discretionary spending, or spending on necessary purchases. You know, paying for food and rent and electricity and so forth. Things which, if you chose not to pay for them, you'd end up in an alley somewhere, possibly dying of plague.
Examples of discretionary items include (as hard as it may seem that these things are not strictly necessary): t-shirts with superhero and/or band logos on them, Netflix accounts, fitted baseball caps, beard-trimming supplies, hot rock massages, and trips to Cancun.