It is often helpful to look at different translations of a Bible passage if the meaning in the King James Version is not clear to us:
Isaiah 50:4 The Lord GOD hath given me the tongue of the learned, that I should know how to speak a word in season to him that is weary: he wakeneth morning by morning, he wakeneth mine ear to hear as the learned. (KJV )
RSV The Lord GOD has given me the tongue of those who are taught, that I may know how to sustain with a word him that is weary. Morning by morning he wakens, he wakens my ear to hear as those who are taught.
NLT The Sovereign LORD has given me His words of wisdom, so that I know how to comfort the weary. Morning by morning He wakens me and opens my understanding to His will.
A word in season means that he knows how to say "just the right thing" in accordance with God's will and word.
I hope you have found this helpful.