This is the 9082nd most frequent Russian word.
смс
SMS (text message).
Here, 'смс' is used as a noun referring to a text message via a phone.
Я отправил смс другу.
I sent an SMS to my friend.
'смс' is again a noun, indicating text messages, but this time focusing on the action of writing or composing them.
Она часто пишет смс на ходу.
She often writes SMS messages while walking.
In this context, 'смс' is used to describe the medium (SMS) through which the notification is received.