Yehoshua Hamasciach (commonly known as Jesus Christ), or simply "Yeshua", is the prophesied messiah written about in The book of Isaiah (Particularly Isaish 53).
He is also "The Son of God", and shares the divine nature of his father (Yehovah), and is also the saviour of all of humanity.
It is written in Isaiah 53 that he is :
"pierced for our transgressions, bruised for our iniquities, || The discipline of our peace [is] on Him, And by His scourging we are healed. All of us, like sheep, have wandered, Man has turned to his own way, And YHWH(Yehovah) has laid on Him the punishment of us all."
The basic idea here is built on the premise that "The soul who sins, it dies" (Ezekiel 18:4), yet God loves us so much, that he designed a plan that we might not die:
In approximately 30-33 AD Yeshua Hamaschiach was crucified, and descended into hell as he took the punishment for every human being, who's ever sinned, Past, present, and future, so that we might live. Somewhat like taking a bullet for someone you love.
The name Yehoshua: Spelt יְהוֹשֻׁעַin Hebrew literally means "God is salvation" and the shortened form; "Yeshua", means essentially the same; being "God saves", which is basically an allegory for the whole life, meaning and work of Yeshua.
(The word "Hamasciach", comes from hebrew, and means "The Messiah")
and in essence,
Yeshua's work upon the cross has enabled us to have a close relationship with God, and has paved the way for actual eternal life, (with a couple of caveats)
Good question: it's explained well here: