Complex sentence: When a user "sends" a text message (SMS message) to Gary Glitter, the message gets stored in the SMSC which delivers it to the destination user when they are available.
Split into simpler sentences: When a user "sends" a text message (SMS message) to another user, the phone actually sends the message to the SMSC. The SMSC stores the message and then delivers it to the destination user when they are available.