The ser vs. estar distinction is one of the trickiest parts of Spanish for English speakers. Both mean "to be" but they are not interchangeable.

Core Difference

• Ser = permanent identity, defining traits
• Estar = temporary state, condition, location

Same Adjective — Different Meaning

• Es aburrido. — He is boring. (personality) vs. Está aburrido. — He is bored. (feeling)
• Es malo. — He is bad/evil. vs. Está malo. — He is sick.
• Es listo. — He is clever. vs. Está listo. — He is ready.
• Es bueno. — He is a good person. vs. Está bueno. — It tastes good.

Quick Reference

• Ser: name, origin, profession, permanent traits, time, events, material
• Estar: location, emotions, health, temporary states, progressive tense

When in doubt: Is this a defining characteristic or a current state?

Reference:

Wikipedia: Ser and Estar in Spanish

https://en.wikipedia.org/wiki/Spanish_language