The Covenant of Redemption...

...is an inter-Trinitarian covenant, between the Father, the Son, and the Holy Spirit before the foundation of the world (Ephesians 1:3-14).

...took place in eternity and is the plan by which election would be elective.

...is called a decree, rather than a covenant, by some theolgians. Covenant Theologians simply say, “You really can’t understand that decree, especially as it regards to our redemption, until you understand the covenant aspect of it.” It is that eternal covenant - that covenant which is prior to time, in which the Son undertakes to be our surety and our mediator and the Father undertakes to give to the Son all the elect because of the Son’s perfect obedience. Christ gives His word prior to Creation to purchase His bride with His own blood (Revelation 13:8).

...is the agreement beyond time and within the Godhead that the Father would appoint the Son, Jesus Christ, to live a perfectly acceptable substitutionary life and die an undeserved death on behalf of, and as a covenantal representative for, those who would sin but would trust in Christ by the power of the Holy Spirit as their covenantal substitutionary representative.

Berkhof defines it this way, “the Covenant of Redemption is the agreement between the Father giving the Son as head and redeemer of all the elect and the Son voluntarily taking the place of those whom the Father has given Him.” And so the Father, foreseeing the fall which He ordained, in His grace effects a covenant with the Son in which He gives all the elect to the Son and the Son says I will take their place (Here are a few places to read about this covenant: Psalm 2:7-9; 40:7-9; 89:3; John 5:36; Hebrews 10:5-7). The Holy Spirit applies Christ's work to the elect through regeneration - making the dead in trespasses and sin alive (Ephesians 2:1-5).