Trails are exactly what you think they are. Using the tool leaves a trail behind the entity that can be pre-set or custom made. They fade out after a short time. You can put trails on almost everything!

1. Press and hold Q to open spawn menu
2. Go to Tools>Render>Trails
3. Adjust the settings to your liking
4. Let go of Q to leave spawn menu
5. Point at an object and fire to add a trail

Well done! Now you know the basics of trails. Go try it out and experiment this awesome tool.