The adjective "τρομερός (masc.), τρομερή (fem.), τρομερό (neut.)" has a double meaning:
1. awful, frightening, terrible
2. amazing, awesome.
The context guides us to decide which meaning fits in each sentence.
"Έχω τρομερό πονοκέφαλο!" = I have a terrible headache!"
"Έχει χιλιάδες θαυμαστές. Είναι τρομερή ηθοποιός!" = She has thousands of fans. She is an amazing actress!"
The two meanings of the word are actually related. "τρομερός" comes from the verb "τρέμω" = tremble. Tremor may be caused by fear or excitement.