Rashi: It refers to not taking T'rumos and Ma'asros from one's crops.

Rashbam: It refers to desecrating Terumah and T'rumas Ma'aser by eating them be'Tum'ah.

Targum Yonasan: It refers to eating Terumah and T'rumas Ma'aser be'Tum'ah.

Yevamos, 89b: It refers to separating from bad on to good ¹ - and it teaches us that what he did is effective; otherwise why would it be considered a sin?

Bechoros, 26b: It refers to Kohanim and Levi'im who assist the owners in the granary and in the abatoir, and who accept T'rumos and Ma'asros in exchange. ²

T'rumos, 1:5: It refers to people who eat from Ma'aser which the owner has given the Levi whilst still in its stalks - before the Levi has separated T'rumas Ma'aser. ³

Rashi: It implies that someone who fails to separate the Matnos Kehunah will bear a sin, and 'someone who desecrates the Kodshim, will die!' ¹