Learn Turkish: En hafif kuş o. - That is the lightest bird.

In Turkish, to say "most" or add "-est" to an adjective, you simply place the word en right before it. Because hafif means "light", en hafif means "the lightest". Notice that the adjective itself doesn't change at all!

Does hafif mean light in weight or light in color?

Hafif strictly means light in weight. If you want to talk about a light color, you would use the word açık (which also means "open"), like açık mavi for "light blue".

Where is the verb "is" in this sentence?

There isn't one! In Turkish, simple sentences using "to be" in the present tense often don't need a visible verb for "is" or "are". The sentence is literally "The lightest bird that," and the "is" is automatically understood by the listener.

Why is the word o (that/it) at the end of the sentence?

The standard textbook word order would actually be O en hafif kuş (That [is] the lightest bird). However, spoken Turkish is very flexible. Moving the subject o to the end makes the sentence sound natural and puts the emphasis right where the speaker wants it: on the description En hafif kuş.