So they want to put it in a regular, taxable account when they have access to a IRA of some kind?
Basically, for a taxable account you use money on which you've already paid taxes, and invest it, and then you pay taxes on any money your investment makes. So you work to earn $1700, and get paid $1250 for that after taxes are paid, and then you invest it. It grows to $2400 and you take it out, paying $200 in taxes on the $1150 in growth, walking away with $2200. So it cost you $1700 in income/working to put $2000 in your pocket.
In a Roth, you also work for $1700, pay taxes on that and have $1250 left to invest, just as in that first example. But now pay not taxes on the increase, so you keep all $2400 when you pull it out, paying no taxes on the $1150 in growth. So your $1700 in work has bought you $2400 instead of $2000.
With a traditional IRA, the money you invest is taken out of your pay *before* income taxes are collected. So your $1700 in work puts $1700 in your investment, instead of the $1250 in the above examples. You pay taxes on the growth/increase, however. So your $1700 grows to $3300 when you take it out. You pay taxes on that entire $3300, paying about $500 in taxes, walking away with $2800. So your $1700 in work gets your $2800.
As you can see, either kind of IRA (Roth or traditional) gets you more than a regular taxable account because you don't pay taxes on either the growth or on the income you invest, as opposed to paying income taxes on all of it.
My examples show the traditional being better, but that's just based on easy to do in my head math as far as tax rates. It all depends on whether you think your tax rate will be higher now, or later, and that's something that's very difficult to know. There are ways to make an educated guess, including current income/rates, and your plans for the future. But either one is going to be a better choice than not using a tax advantaged account at all.
If you are looking for an easy way to explain it, it's in that "as you can see" paragraph. Tax advantaged accounts allow you to not pay income taxes either on the money you invest, or the growth in your invested money. Taxable accounts force you to pay taxes on both of those things.