In the US, every trademark application must declare a filing basis: either the mark is already being used in US commerce, or the applicant has a genuine intention to use it there in the future.
A use-in-commerce application (basis 1a) is for marks already in active commercial use in the US market at the time of filing. The applicant submits a specimen showing the mark in use alongside the application.
An intent-to-use application (basis 1b) is for marks not yet in US commercial use. The applicant declares a genuine intention to use the mark in the US. After the application passes examination and the opposition period, the USPTO issues a Notice of Allowance, and the applicant then has a set window to submit a Statement of Use demonstrating actual US commercial activity.
Most IP offices outside the US do not make this distinction; applications can be filed without declaring a use basis.