From that pic, it will be a wooden joist that has been plastered over. Depending on the age of the house, it might be plasterboard or L&P for the wall above, and it looks like the plaster over the joist is different or thinner. It might also be cracking if you have dancing elephants or something in the room behind.
There might once have been a skirting or cornice or other ornamental feature that has been removed.
It’s a 1930s house. The room behind is my toddlers room who loves to bang everything so that may not help! Nothing massive to worry about then? Just keep an eye on it and make sure it doesn’t get worse?
Next time you decorate, strip off the wallpaper and see what's behind. My hunch is there used to be a painted board, like on the adjacent landing edge.
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