To squirt is to abruptly squeeze a liquid out of something. When you spend the day at the beach, you might squirt sunblock on your arms or squirt ketchup on the fries you bought at the snack bar.
Octopuses and squid use their ability to squirt black ink to confuse their predators. Most kids enjoy using squirt guns to squirt each other with water. In both cases, the discharge itself can be called a squirt. If someone calls you a squirt, however, they mean that you're an insignificant and possibly annoying person: "Get lost, squirt! You're bothering me!"