Al-Quran.Sura 23: Verse 2

ٱلَّذِينَ هُمْ فِى صَلَاتِهِمْ خَٰشِعُونَ

They who are during their Salat humbly submissive.

যারা নিজদের সালাতে বিনয়াবনত।

Humbleness is a deep act of understanding which doesn’t mean only to have focus and concentration in our Salah but much beyond that.
Many people who meditate would have good focus and concentration but not necessarily they’ll have humbleness.

We should be humble before Allah, have fear of His punishment and have hope in His mercy and mostly being in awe by the remembrance of Allah, that should be shown in our actions of daily lives.

Being humble, learning to treat people the same respectful way whether it’s the king or the popper, controlling our anger, being approachable for any good cause by anyone, being helpful every manner we could, refraining ourselves from the acts those are displeasing to Allah are all forms of humbleness.

May Allah make us all of those who develop humbleness in their salaah and daily lives and make our lives successful in this world and the hereafter.

Note: Salat means to follow the commandments of Allah/ maintain the commitments/connected with Allah and many more not only prayer. The meanings of the Salat applied in the Quran according to context of the verses. And. Dua means prayer.