What causes the smear below the band in electrophoresis?

When the PCR product was electrophoresed, such a band was visible and smear was seen under the band.

Cycle conditions were (1) 94.0°C for 3 min, (2) 94.0°C for 10 sec, (3) 55.0°C for 10 sec, (4) 72.0°C for 7 sec, (5) 72.0°C for 1 min, and (6) 20.0°C for 10 min.

The number of cycles is 40 cycles from (2) to (4).

The swimming time was 15 min.

Gels were 3% agarose gels.

I would like to know what causes such smears and how to improve them.