This trigger will be fired when an incoming sms is received.
You can specify a particular contact that the incoming sms should come from, or alternatively you can specify that the message should come from Any Contact, a Non-Contact or Any Number.
You can also compare the content of the incoming text so that the trigger is only activated when the text of the sms matches or contains the required text.
Note that if this trigger does not work, it may be that your messaging app uses the 'RCS' protocol instead of actual SMS. These are not detectable directly by MacroDroid. An alternative is to use the notification received trigger.