It's your mind, your will, and your emotions.
Let's look at 1 Peter 1:22.
"Since you have purified your souls in obeying the truth through the Spirit in sincere love of the brethren, love one another fervently with a pure heart"
So how does our soul become purified?
By obeying the word of God.
In John 14:15 Jesus said, "If you love me keep my commandments."
We obey God because we love Him.
When you obey God your soul will become purified.