Sitting is part of the continuous form of the verb 'to sit'
The continuous from is formed by 'to be' +sit
(verb) +ing.
e.g.
Present
- I am sitting, we are sitting
- you are sitting
- he/she/it is sitting, they are sitting
Past
- I was sitting, we were sitting
- you were sitting,
- he/she/it was sitting, they were sitting
Future
- I/we/you/he/she/it/they will be sitting.