The Hebrew phrase Adonai Tzeva'ot (יי צְבָאות) is one of the many names of God. It is often translated as "Lord of Hosts." The Hebrew word tzeva'ot (צְבָאות) can also be translated as "armies," giving the idea of God as a commander of all heavenly forces. The name is not used in the Torah (the first five books of the Hebrew Bible) but appears frequently in the later prophetic works, including Haggai, Zachariah, and Malachi, as well as many times in the Psalms. Various commentators and interpreters have described God's "armies" or "hosts" as consisting of angelic beings, but have also suggested that all the elements of creation - atoms, molecules, etc. - are what is meant by "hosts." This latter idea alludes to God as the commander of all available physical particles that can come together to form tangible entities. Consider the following verse from Genesis 2:1, which describes a completed creation:

The heaven and the earth were finished, and all their array (וְכָל־צְבָאָֽם) - all the atoms, molecules, and particles had been arranged in the right order to result in the physical world as we know it.

The name Adonai Tzeva'ot first appears in the book of Samuel, when a man named Elkanah heads to Jerusalem with his two wives, Hannah and Penina, to make sacrifices to God - or, specifically, to Adonai Tezva'ot (1 Samuel 1:3). Hannah, who is bereft over her childlessness, will remain behind to address God personally. She says:

“Adonai Tzeva'ot, if You will look upon the suffering of Your maidservant and will remember me and not forget Your maidservant, and if You will grant Your maidservant a male child, I will dedicate him to Adonai for all the days of his life; and no razor shall ever touch his head.” (1 Samuel 1:11)

Rashi, a medieval French commentator, explains that Hannah's use of this name was no coincidence; she seemed to be intentionally commenting on God's power:

She said before Him: 'O Ruler of the Universe, You created two hosts in Your world. The heavenly beings do not multiply, neither do they die, while the earthly beings both multiply and die. If I am of the earthly beings let me multiply, and if I am of the heavenly beings let me not die.' (Rashi on 1 Samuel 1:11)

The Talmud (Berachot 31a) explains that this was the first time any human being had called God by this name:

Rabbi Elazar explains that Hannah said before the Holy One, Blessed be God: 'Ruler of the Universe, are You not the Commander of the Hosts, and of all of the hosts and hosts of creations that You created in Your world, is it difficult in Your eyes to grant me one son? A parable: To what is this similar? It is similar to a flesh and blood king who made a feast for his servants. A poor person came and stood at the door. He said to them: Give me one slice of bread! And they paid him no attention. He pushed and entered before the king. He said to him: My lord, the King, from this entire feast that you have prepared, is it so difficult in your eyes to give me a single slice of bread? (Berachot 31a)

Therefore the phrase Adonai Tzeva'ot is meant to connote God's abundance of power, whether that be as commander of an army of angels or the master of all the ingredients of creation, as well as to hint that a God so powerful should not find it difficult to grant us small blessings.