What does the "Math processing error" red button means

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I noticed that while I was running some code in a wroksheet in sagemathcloud, a button with red letters saying: "Math processing error" appeared in the toolbar.

when I hover over it, a tooltip appears saying "define symbolic variable".

If I press the button, the following is inserted in the current cell:

%var x, theta

what does this mean? I didn't get any error messages in my code