The watermark will appear in the Free and Creator tier subscriptions if you use an effect or feature that is not available in that plan. You can see which effects are part of your plan by looking at your Effects panel, and noting which effects have the word 'Upgrade' next to it. After upgrading, the watermark will be removed from your future exports.
If you do not wish to upgrade at this time, please remove the effect from your project, and the watermark will be immediately removed.
For a full comparison of the effects and features available in each plan, please see our blog post.