People who look for something or watch something carefully.
Spotters are usually good because they help find things or keep things safe.
Examples:
The bird spotters look for birds in the trees.
The lifeguard spotters watch the people in the water.
The treasure spotters find the hidden gold.
The plane spotters see the big planes in the sky.
The spotters find the lost dog in the park.
The spotters keep the city safe from bad things.