The verb estar (conjugation) means "to be". It is irregular in the present tense.

Estar is used to signify the location of an object:
El perro está al lado del gato.
The dog is next to the cat.

La escuela está al lado del correo.
The school is next to the post office.

Estar + past participle is used to indicate a resultant condition:
Estoy cansado.
I am tired.

Estamos despiertos.
We are awake.

Estar is used to express a changeable state or condition:
María está enferma.
María is sick.

Ella está enojada hoy.
She is mad today.

Hoy estoy enfermo.
I am sick today.

Estar is used to express an action in progress (progressive tense)

Estoy escribiendo.
I am writing.
Juan está escuchando.
Juan is listening.

Present tense sentences with estoy
Present tense sentences with estás
Present tense sentences with está
Present tense sentences with estamos
Present tense sentences with están