着 is a particle that explicitly marks that an action is ongoing. It’s not used nearly as much as 在 is in everyday speech, though.
First, you can use it after a verb to show that something is in the continuous state expressed by that verb.
|The restaurant’s door is open.|
Remember, this is different than the “change of state” particle 了. Note the difference:
|The restaurant’s door is now closed.|
Another common verb you see this particle with is 关 “to close.”
|The car’s windows are all closed.|