The pro rata rule comes into play when you have existing pre-tax money in other traditional IRAs.
The IRS treats all your traditional IRAs as one big account when you do a conversion. For example, if you have $93,000 in pre-tax money and $7,000 in after-tax money in your traditional IRAs, and you want to convert $7,000 to a Roth, 93% of that $7,000 will be treated as coming from the pre-tax portion, even if you intended to convert only the after-tax money.
This means that even though you made after-tax contributions, a significant portion of the conversion will be taxed as ordinary income because it's coming from your pre-tax IRA balance.
The only way to completely avoid the pro rata rule is to have no pre-tax money in your traditional IRAs before doing a Roth conversion. This can be achieved by rolling over your pre-tax IRA money into your employer's retirement plan (if allowed).
