Trademark conversion refers to the process of changing the filing basis of a trademark application from one type to another. In the United States, a trademark application can be filed on either an "intent-to-use" basis or a "use-in-commerce" basis.
If an applicant initially files their application on an "intent-to-use" basis, they must provide evidence of use in commerce before their trademark can be registered. If they later decide to file on a "use-in-commerce" basis instead, they can request to convert their application.
Likewise, if an applicant initially files their application on a "use-in-commerce" basis but later decides they are not yet using the trademark in commerce, they may be able to convert their application to an "intent-to-use" basis.