ACH stands for Automated Clearing House and (when enabled) it allows clients to link their bank accounts to the client portal as a payment option. This is an alternative to using credit / debit cards and reduces processing fees.
Using the new "Prefer ACH" option means that you can encourage clients to use ACH (as opposed to entering a credit card), while still providing an option to add a credit card, albeit a less obvious option.
Switching to "Prefer ACH" means that your client sees the option to enter their bank account details as a preference to adding a credit card.
If the client decides they do not want to add their bank account details, there can still click "Show all options" where they will find the option to add their credit or debit card details.
Click on the presentation for more information.