“Jesus Christ the same yesterday, and to day, and for ever.” Hebrews 13:8 (KJB)
Jesus Christ the second member of the Holy Trinity, the Son of God, God the Son is Immutable just as the whole of the Godhead.
This verse is referring back to verse 7, and prepping the reader for verse 9. This is the same Jesus Christ who worked in the Old Testament; died, was buried, and rose again according to the gospels; ascended into Heaven; and He lives forevermore.
The following commentary is from the Jamieson, Fausset, and Brown Commentary…
“This verse is not, as some read it, in apposition with “the end of their conversation” (Heb_13:7), but forms the transition. “Jesus Christ, yesterday and to-day (is) the same, and (shall be the same) unto the ages (that is, unto all ages).” The Jesus Christ (the full name being given, to mark with affectionate solemnity both His person and His office) who supported your spiritual rulers through life even unto their end “yesterday” (in times past), being at once “the Author and the Finisher of their faith” (Heb_12:2), remains still the same Jesus Christ “to-day,” ready to help you also, if like them you walk by “faith” in Him. Compare “this same Jesus,” Act_1:11. He who yesterday (proverbial for the past time) suffered and died, is to-day in glory (Rev_1:18). “As night comes between yesterday and to-day, and yet night itself is swallowed up by yesterday and to-day, so the “suffering” did not so interrupt the glory of Jesus Christ which was of yesterday, and that which is to-day, as not to continue to be the same. He is the same yesterday, before He came into the world, and to-day, in heaven. Yesterday in the time of our predecessors, and to-day in our age” [Bengel]. So the doctrine is the same, not variable: this verse thus forms the transition between Heb_13:7 and Heb_13:9. He is always “the same” (Heb_1:12). The same in the Old and in the New Testament.”