The book of Hebrews makes a contrast between Christian antitypes and "a shadow of heavenly things" that was associated with the Torah (Hebrews 8:1-5). As our High Priest according to the manner of Melchizedek, Jesus (in his role as PRODROMOS) entered within the curtain of the greater Most Holy or SANCTUM SANCTORUM (cf. Leviticus 16:1-2, 12, 15). What, however, is the "curtain" wherein Christ took the value of his precious blood?
According to Hebrews 10:20, the curtain represents Christ's flesh; hence, by dying and being resurrected to spirit life, the Lord Jesus entered the curtain in that he no longer had flesh as a barrier, but now dwelt as an immortal or incorruptible spirit being (1 Peter 3:18; 1 Corinthians 15:45). It seems that 10:20 would make a good point to share with those who believe that Christ still has a fleshly body or that he has not entered within the curtain (figuratively speaking).
At any rate, the main point of the text seems clear enough. Jesus entered within (EISERXOMENHN EIS TO ESWTERON) the antitypical Most Holy (Holy of Holies) after he transcended his fleshly state:
"ESWTERON (#2278) within, inside of" (Rogers and Rogers Linguistic and Exegetical Key, page 529). See Acts 16:24.
BDAG Greek-English Lexicon notes: "In TO ESWTERON TOU KATAPETASMATOS, E. funct. as prep. w. gen. (cp. 1 Km 24:4) what is inside (= behind) the curtain, the Holy of Holies Hb 6:19 (Lev 16:2, 12, 15)."