No one knows what it is. Destiny's story has changed a bit since D1 but grimoire/lore cards from back then seem to reveal that the Traveler isn't some merciful, benevolent (or innocent) being. It's actually responsible for wiping humanity out ("the collapse") in the first place, and, using its advanced technology, bringing some of the population back to life as "guardians" to fight whatever beings threaten it (Cabal, Fallen, Hive, "the Darkness," etc). So, it's a bit strange that we worship it as a God, right? When it's responsible for genocide, and essentially enslaving us to fight for it... The problem is hardly anyone has a memory of their past lives, so most only have memories starting from when they were resurrected. Why does the Traveler need warriors to protect it? Well, it's likely done this whole genocide thing before with other races. Supposedly (could be remembering this wrong), it was occupying the Fallen homeworld, and they too worshipped it as a God. At some point, it left (to humanity) and the Fallen I guess hate us for that. They've attempted to recreate their own Traveler in a sense, through Servitors but obviously they don't compare. The Hive (I think) are a creation of the Traveler gone wrong. Cabal are just, Cabal. Some of those details could be wrong, so anyone feel free to correct me. It's been a while since I read about this stuff hehe.