“Ser” y “Estar” both mean “to be,” but their applications are different.
Even though entire chapters in grammar texts have been written on the use of these verbs, here is the condensed explanation:
Ser is used when referring to characteristics and/or origins.
Estar is used for conditions and locations.
For example: ¿Cómo está Pedro? vs. ¿Cómo es Pedro?
In the first case, ¿Cómo está Pedro? asks about Pedro’s condition, and could be answered with “Pedro está bien,” or “Pedro está mal,” or “Pedro está ocupado,” or “Pedro está cansado.” All of these describe Pedro’s condition, telling us that Pedro is well, unwell, busy, or tired.
In the second case, “¿Cómo es Pedro?,” The question is asking what Pedro is like as a person, and is asking about his personality and/or his physical characteristics, and could be answered, and could be answered with “Pedro es alto,” or “Pedro es delgado,” or “Pedro es inteligente,” or “Pedro es trabajador.” All of these describe Pedro’s characteristics, telling us that Pedro is tall, thin, intelligent, or hard-working.
If I use “ser” and ask “¿De dónde es Pedro?.” I am asking about his origin, i.e., where he is from. This could then be answered with: “Pedro es de Panamá.”
If I use “estar” and ask “¿Dónde está Pedro?,” I am asking where he is at this moment. This could be answered, “Pedro está en la biblioteca.”
The use of “ser” or “estar” can create differences of meaning, such as the following:
“Anita es feliz,” using “ser” to describe a characteristic means that Anita is a happy person, that she is happy by nature.
“Anita está feliz,” using “estar” to describe a condition means that Anita is happy for a condition, or a specific reason, such as winning the lottery, getting good grades on her final exams, going to eat at her favourite diner.