When working with inkscape, keep an eye on the bottom indication line.
Selecting an object on screen, it shows if there is a filter added.
This case, the selected object depicting a faucet is a group, and it has a filter added which is named ”rusty”.
Note how the selection box is not touching the edges of the visual boundaries of the object.
That is due to by default visual bounding box is set.
Certainly it doesn't make sense at this point, so better rephrase that as if not geometric bounding box is set, something interferes with the visual cue when filters are involved.
It also often helps to understand a document's structure to switch rendering modes.
With no filters rendering mode usually the filtered objects are sticking out.
To toggle between rendering modes, press Ctrl+5 on the numpad.
For adding a filter to a selected object, the most straightforward way is going by the filters menu on top.
That is containing quite a lot of filters built in to choose from.
However, due to its regular menu item limitations, it's not a very helpful for choosing.
There is no visual indication built in of the filters affecting a preview object's appearance, nor there is an option to go through all like when previewing a font and styling on a text object.
It's all pure luck to find a suiting filter by first pick.
Either it takes applying the filters one after another on the object selected and undoing every time the outcome is not right, or having a sneak peek on a guide image where all the filters are displayed in action.
The current (not yet up to date) manual is taking 19 pages to showcase them all.
If inkscape was installed with the example files, check the filters.svg located in inkscape/share/examples to see an all-in-one action shot.