What does ECG report saying t wave inversion in III & avf means.

ECG . T wave inversion in leads 2 and 3 doesn't necessarily mean anything significant. It depends on other factors and if the only thing mentioned is t wave inversion likely nothing of concern.
Nonspecific. The t wave is more commonly upright in those leads; when it is upside down (a negative deflection), it is inverted. T wave inversion in leads iii and avf can represent normal findings, but in the appropriate clinical setting, can indicate that further evaluation of the heart is needed.