This is the 3831st most frequent Spanish word.
sentarse
To sit down.
The word 'siéntate' is the imperative form of 'sentarse' used to address someone informally and make a polite request for them to sit down.
Por favor, siéntate mientras preparan la comida.
Please sit down while they prepare the food.
In this sentence, 'sentarme' is the reflexive infinitive form of the verb 'sentarse', indicating that the subject enjoys the action of sitting performed by themselves.
Me gusta sentarme junto a la ventana.
I like to sit by the window.
Here, 'se sientan' is the present indicative conjugation of 'sentarse' in the third person plural, reflecting the habitual action they perform.