Tony is the Romeo equivalent in West Side Story. Though he was once a member of the Jet gang, Tony no longer affiliates with them and is more a lover than a fighter by nature. However, he still cares for his friends in the gang.
This loyalty is a strong trait of Tony's, both a great virtue and a catalyst to impulsive violence. He becomes devoted to Maria, willing to cross social taboos and face danger just to be with her. He kills Bernardo when Bernardo accidentally kills Tony's pal Riff during a street fight. His great love for Maria also turns destructive when he believes she's been killed by Chino; he proceeds to run through the streets, aiming to have Chino kill him as well.
Another important trait of Tony's is idealism. He is a romantic despite the squalor of his daily life and the violence around him. Despite the opposition he and Maria face, he believes they can run away together and marry. When reality comes crashing around him, he becomes suicidal.
The song "Maria" focuses on Tony's falling for Maria after meeting her at the school dance. It is a meeting that changes his life. The lyrics and soaring orchestration emphasize how important this encounter is for Tony. He even goes as far as to compare saying her name to a religious experience:
Say it loud and there's music playing,
Say it soft and it's almost like praying.
(This comparison is actually a nice callback to Romeo and Juliet, where Romeo calls Juliet "sweet saint," and the two compare their flirting to prayer.)
"Maria" shows how Tony now has a direction in life, something he felt he was missing after no longer being a part of the Jets. Now, he can channel his energies towards a romance with Maria—something positive, unlike the violent activities of the gangs.