A steward is someone who takes care of the grounds or animals of a particular place. If your parents are always traveling and you have to take care of your six younger siblings, you probably feel like the steward of the house.
You might have heard the term "stewardess," a female flight attendant, but the name has trickled out of use in favor of the gender-neutral "flight attendant." A steward is the male version of that. Anyone who takes care of places or people is a steward. You can be a steward of artifacts from an architectural dig. You can also be elected to be the steward of your co-workers retirement fund. Either way, someone's trusting you with their stuff.