What is spiritual communion?

St. Thomas Aquinas explained that Spiritual Communion is “an ardent desire to receive Jesus in the Holy Sacrament and a loving embrace as though we had already received Him.” This is an example:

My Jesus,
I believe that You are present in the Most Holy Sacrament.
I love You above all things,
and I desire to receive You into my soul.
Since I cannot at this moment receive You sacramentally,
come at least spiritually into my heart.
I embrace You as if You were already there
and unite myself wholly to You.
Never permit me to be separated from You. Amen.