It means : The salutations and peace upon you O Messenger of Allah.
'It is usually said by many Sunni brothers. It is incomplete form of salutations because it must mention the Progeny of the Prophet along with him as the Prophet Muhammad (SAWA) said: لا تُصَلّوا عَلَيَّ الصلاة البتراء Do not salute me with a cut off salutation. They aske him: What is the cut off salutation? He replied: To salute me with saluting my Progeny. ( al-Qawl al-Badee' by al-Sakhaawi; page 45 and al-Sawaa'iq al-Muhriqa by Ibn Hajar al-Haytami; page 225, and Yanabee' al-Mawaddah by al-Qondoozi al-Hanafi ;2:434).
It is always recommended to salute the Prophet Muhammad (SAWA) and his Progeny Ahlul Bayt (AS) any time including before and after Azan and before and after Prayers.