it means, that instead of asking for your money, that they need to keep their servers running, they find other sources of income. nothing is free.

of course we can only speculate what other sources those are, but selling user-data to advertisers, government agencies or "think tanks" has happened.

in this case the following can be found in creditkarma's website:

When you take an offer through Credit Karma (like for a credit card or a loan), we usually make some money from one of our partners (like the bank that issues the card or the lender who funds your loan).

We'll even crunch the numbers and give you personalized recommendations.

this means, that they use the user-data in-house to improve their own credit card offerings. you get your free taxes, they win you as a prime advertising target.

there's more in the privacy policy: https://www.creditkarma.com/about/privacy-20180531

We may share information about you with service providers or agents in order to have them perform on our behalf any of the activities we are permitted to conduct under this Privacy Policy and our Terms of Service. This includes things like designing or operating features, helping us research and test feature improvements, identifying offers or personalized recommendations, performing analytics and marketing our Services.

ianal, but you definitely are the product here. the only reason they offer free taxes, is to get more information about people to improve their money making side of the business.