St. Thomas Aquinas described it as " an ardent desire to receive Jesus in the most holy sacrament and lovingly embrace him” at a time or in circumstances when we cannot receive him in sacramental Communion. Many saints who were masters of the spiritual life would incorporate making a spiritual communion as part of their daily lives especially when their desire for union with God was more than just once a week or if circumstances prohibited them from receiving the Eucharist.

Here is a prayer that you can say to make a spiritual communion. 

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.