After the opening تكبير we recite دعاء الاستفتاح - the opening supplication. The most common one starts with the phrase سبحانك اللهم. The word سبحانك is comprised of سبحان and كَ meaning ‘you’.
The word سبحان appears in the phrase سبحان الله which is usually translated to mean ‘Glory be to Allah’, but linguistically it originates from the meaning of distance and remoteness. So the implications of saying سبحانك are actually twofold:
Firstly, we are distancing our own hearts and thoughts from assuming or attributing any deficiency or evil to Allah.
And secondly, we are also removing and distancing Allāh Himself from every wrongdoing, deficiency, shortcoming and inappropriateness, and this is a way of glorifying His exaltedness.
An interesting point to note is that the word سبحان comes from the root meaning س ب ح which refers to the act of swimming, and the scholars have commented that just as a person would drown physically if they found themselves immersed in the ocean and they stop swimming, so a person drowns spiritually if they stop praising and glorifying Allah while they are immersed in the ocean of this dunya.
The word اللهم is simply a way of saying يا الله, where the initial يا has been omitted and replaced with a م meem at the end, a letter possibly chosen because it involves the lips being gathered together, as if the servant is gathering his heart upon his Lord as he calls out to Him in supplication.
So when we say سبحانك اللهم what we are essentially saying is: O Allah, I have gathered my heart to focus on You alone as I call out to you, recognising Your absolute perfection and distancing You from every deficiency and shortcoming.