I believe that this refers to the mystery of God's will which is explained in verses 9-10, which is the summing up (gathering together again into one or reuniting under one Head)  of all things in Christ, both things on the earth and things in the heavens. Believers have an inheritance or an allottment in this plan. God bless.

1:9
He made known to us the mystery of His will, according to His kind intention which He purposed in Him

Ephesians 1:10
with a view to an administration suitable to the fullness of the times, that is, the summing up of all things in Christ, things in the heavens and things on the earth. In Him

Ephesians 1:11
also we have obtained an inheritance, having been predestined according to His purpose who works all things after the counsel of His will,

Ephesians 1:12
to the end that we who were the first to hope in Christ would be to the praise of His glory.

God bless.