Islam means submitting to Allaah, worshipping Him alone and obeying Him, and shunning shirk (polytheism, associating others with Allaah) and those who believe in that. This is the religion which Allaah approves for His slaves. Allaah says (interpretation of the meaning): 

“Truly, the religion with Allaah is Islam”

[Aal- ‘Imraan 3:19] 

“And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers”

[Aal ‘Imraan 3:85]

The person who enters this religion is called a Muslim (one who submits), because he has submitted (aslama) to all the rulings that have come from Allaah and from the Messenger of Allaah (peace and blessings of Allaah be upon him). Allaah says (interpretation of the meaning): 

“And who turns away from the religion of Ibraaheem (Abraham) (i.e. Islamic Monotheism) except him who befools himself? Truly, We chose him in this world and verily, in the Hereafter he will be among the righteous.

When his Lord said to him, ‘Submit (i.e. be a Muslim)!’ He said, ‘I have submitted myself (as a Muslim) to the Lord of the ‘Aalameen (mankind, jinn and all that exists).’” [al-Baqarah 2:130-131] 

“Yes, but whoever submits his face (himself) to Allaah (i.e. follows Allaah’s religion of Islamic Monotheism) and he is a Muhsin ( a doer of  good who does good deeds solely for the sake of Allaah) then his reward is with his Lord (Allaah), on such shall be no fear, nor shall they grieve”

[al-Baqarah 2:112].