The heart has been associated with love since ancient times. The Christian cross is alsoa symbol of love. The Heart Cross helps s focus thoughts on this love.
The Heart Cross represents and reminds humanity of the love of Jesus, and to remind people to devote themselves to the love shown by God.
Therefore the heart symbol can represent the love from God, or the love given to God, which in turn means the love given to other people. In this context it is often referred to as the Sacred Heart.