This is the 2128th most frequent Japanese word.
座っ
"座っ" is the stem form of the verb "座る" (suwaru), meaning "to sit."
The word '座っ' in this context is the stem of the word '座る' ('to sit') conjugated into the present progressive form '座っています', indicating an ongoing action of sitting.
私の犬は椅子に座っています。
My dog is sitting on the chair.
Here '座っ' is used in the past progressive form '座っていました', showing the action of sitting that was occurring in the past.
その男の人は道端に座って話していました。
That man was sitting by the roadside talking.
In this case, '座っ' is conjugated into the te-form '座って', which is often used in requests or invitations, as in '座ってください' ('please sit').