What Does It Mean When It Is Said An Animal Is Using Tools?

An animal is said to be a tool user if it uses or fashions an object to attain a means, specifically food.

Apes and monkeys are well-known tool users. They smash open nuts to get at the meats with rocks. Chimps have been seen dipping a stick into termite hills, then licking off the scurrying insects. Primates also fashion tools by bending sticks into more efficient shapes.
Otters are known for using rocks to crack open abalone shells as they float on their backs.
Birds are also know to use tools. Jays select strong, forked branches and use them as a vise to crack nuts. Another jay figured out how to use a piece of newspaper near its laboratory cage to sweep seeds to within eating range. It repeated the trick with a feather, piece of wire, and paper clip. Soon, all of the other birds in the study had learned the trick.

A jay was observed swooping down to eat the meat of nuts cracked open by passing vehicles. Eventually, the bird figured out that it could hasten the process and get more-consistent results by dropping the nuts from an overpass and waiting for cars to pass -- thus using the vehicles as a "tool."

A lab crow was seen using piece of wire to attempt to dislodge food from a tube. When this proved unsuccessful, the bird fashioned the wire into a hook and fished the morsel out.

A captive crow was fed dried mash, which its owners sometimes forgot to moisten. Without prompting, the bird learned to use a small plastic cup provided as a toy to dip into a trough, carry across the room, then empty onto the mash.