Everything is explained on ENSEMBL website : https://www.ensembl.org/info/genome/variation/prediction/predicted_data.html

and here : https://www.ensembl.org/Help/Glossary

Variant impact A subjective classification of the severity of the variant consequence, based on agreement with SNPEff.

Modifier impact variant consequence Usually non-coding variants or variants affecting non-coding genes, where predictions are difficult or there is no evidence of impact.

Low impact variant consequence A variant that is assumed to be mostly harmless or unlikely to change protein behaviour.

Moderate impact variant consequence A non-disruptive variant that might change protein effectiveness.

High impact variant consequence The variant is assumed to have high (disruptive) impact in the protein, probably causing protein truncation, loss of function or triggering nonsense mediated dec