Jihad is an Arabic word and a (usually boys') name, meaning to strive and do one's utmost.
It comes from the root word 'jahada', which means to exert strength and effort.
As a concept in Islam, there are a couple of ways to understand it (please be aware, I'm not a qualified scholar, and Allah knows best).
One is to consider the Greater Jihad and the Lesser Jihad. The Greater Jihad is a person's efforts to live a good life, to strive to avoid sinful/wrong acts and to treat others well. The Lesser Jihad is that of fighting to defend and safeguard the Ummah (the wider Muslim community) and other innocent people from direct threats of harm (eg to defend a country against aggressors in an act of self-defence).
Another way is to consider the four kinds of struggle in the name/cause of God: jihad of the heart (working to be a good-hearted person with good intentions), jihad of the tongue (trying to speak truthfully and kindly, rather than to spread lies or cruelty), jihad of the hand (striving to act justly and avoid doing wrongful acts), and jihad of the sword (fighting to defend those in harm's way).