The Prophet (SAW) said it refers to 4 dhikr phrases:
SubhanAllah: Limitless is Allah in His Glory.
Tasbeeh (saying of SubhanAllah): Extolling the purity of Allah from everything that does not befit Him.
Alhamdulillah: All praise is due to Allah.
Tahmeed (saying of Alhamdulillah): Demonstrating the perfection of Allah in His names, attributes and deeds.
La ilaha ilallah: There is no deity besides Allah.
Tahleel (saying La ilaha ilallah): Dedication of the heart to Allah in His oneness and purify from associating partners with Him.
Allahu Akbar: God is the Greatest.
Takbeer (saying Allahu Akbar): Demonstrating the greatness and majesty of Allah and that nothing is greater than Him.