Rashi: All the Nevi'im who prophesized salvation, they consoled Yisrael and strengthened their weak hands. Chazku is Pi'el (strengthen others). If the verse meant 'be strengthened, for your hands are weak', it would use the Kal conjugation - "Chizku v'Imtzu Al Tira'u" (Divrei ha'Yamim II, 32:7), like one tells an individual "Chazak ve'Ematz" (Yehoshua 1:9). "Chazku" and "Ametzu" are to strengthen others.

Radak #1: Sometimes, a command is not to a particular person, rather, to anyone, like "Re'eh Rei'ach Bni" (Bereishis 27:27). Also here, it is to anyone.

Radak #2: This is said to the Nevi'im, or one who has a strong heart in Galus, and trusts in Kel every day to bring the salvation. He tells them to strengthen weak hands in Galus. Also Targum Yonasan is like this.

Malbim: This commands Nevi'im to strengthen the nation, who despair from redemption. Despair affects three matters. (a) The hands weaken and do not do anything, amidst despair. (b) Even though he is quick to do Hashem's work and Avodah, his knees stumble from the great affliction until he is weary, not from laziness - "u'Virkayim Koshelos Ametzu." (c) Those who invigorate in Avodas Hashem and do not slacken, and did not stumble in affliction - they had serenity - but since they see that time and time have passed, and the Nevi'im's words were not fulfilled, they considered the words of Nevu'ah false; refer to 35:4:1:3.