Those circles are lens flare: reflections bouncing between lens elements (and sometimes filters) when a bright point light source, such as the sun, is in or just outside the frame.

To reduce it:

• Use a lens hood.
• Keep the sun/bright light out of frame when possible, and avoid having it just outside the edge.
• Remove unnecessary front filters; extra glass can add more flare.
• Keep the front element clean.
• Be aware that flare is often more noticeable at smaller apertures (higher f-numbers) and with wide-angle lenses.
• Better lens coatings can help, but flare usually can’t be eliminated completely in harsh light.

To create flare intentionally:

• Include a bright point light source in or near the frame.
• Try smaller apertures and wide-angle lenses.
• Shooting toward the sun, lamps, headlights, or candles can make it more obvious.

As for safety: flare itself does not damage the sensor. The real risk is aiming directly at very bright light sources, especially the sun, for extended periods. Use care when shooting into the sun.