It means it's more safe to hold your currency locally, in a hardware wallet, than trusting an online service to do it for you. One that can be hacked or go out of business, or be a scam. They're not as reliable as large banks (which themselves proved not to be so reliable either)

So it means when you're not actively trading but holding it for years/months.
Of course you can also screw up holding your bitcoins yourself, but then it's your own fault.