How can I remove horizontal lines from my MacBook Air?

There are seldom any home fixes for display issues. There are tests, but whatever they reveal requires professional fixing.

1 — Move the display assembly back and forth on its hinge, watch the screen as it moves. If the articlafa cahnge during movement, that suggests a worn display cable.

2 — Examine the outside of the display assembly for evidence of impact.

3 — Connect an external monitor. If the defects do NOT show on the external, that isolates the issue to the dipsaly assembly. If they repeat on the external, that suggest a logic board fault.

Regardless of what you find, you will need this: Mac Repair and Service - Apple Support

If you don't have an external monitor to connect to the MacBook to test as Allan Jones recommends, an alternate would be to take a screen shot using the Mac's built-in Screenshot app.. If the anomaly shows in the screen shot, chances are good that it's not hardware related.

If the the anomaly does NOT show in the screen shot but you see it visibly on the screen, the issue is more than likely hardware related and will require service.