Learn Turkish: Hedefleri çok zor. - Their goal is very difficult.

It is the possessive suffix meaning "their" (from the pattern -lArI taught in this lesson). Because the last vowel in hedef is e, e-type vowel harmony requires the e/i version of the suffix, making it -leri.

Can hedefleri also mean "his goals" or "her goals"?

Yes! This is a very common double meaning in Turkish. Hedefleri can mean "their goal" (hedef + -leri), but it can also mean "his/her goals" (hedef + plural -ler + his/her -i) or even "their goals". You rely on context to know which one the speaker means.

Where is the separate word for "their" in this sentence?

The word "their" is completely contained in the suffix -leri. You could add the pronoun onların ("their") at the beginning of the sentence for emphasis (Onların hedefleri çok zor), but it is usually dropped in everyday Turkish because the suffix already gives us that information.

How does this sentence say "is" without a verb?

In simple descriptive sentences like this, the "is" is implied. By placing the adjective zor (difficult) at the end of the sentence right after the subject (hedefleri), it automatically translates as "their goal IS difficult."