Lewis (the author) describes Aslan as "good and terrible." how can something be both good and terrible?

In Aslan's case, terrible refers to his look..... his demeanor. For the children, Aslan was beautiful and good, until they saw him in all of his glory..... and fierceness. The children were afraid.... briefly.

People who have not been in Narnia sometimes think that a thing cannot be good and terrible at the same time. If the children had ever thought so, they were cured of it now. For when they tried to look at Aslan's face they just caught a glimpse of the golden mane and the great, royal, solemn, overwhelming eyes; and then they found they couldn't look at him and went all trembly.