The jury struts on a monoplane are small subsidiary struts that join the main wing struts to the wing.
Jury struts are installed to eliminate unwanted resonance and vibration from the main wing struts and also to prevent main strut compression buckling under negative-g loads in the case of high-wing aircraft. They are commonly installed on braced high-wing monoplanes, but are also used on braced low-wing aircraft, where they provide extra positive g-capabilities.
Jury struts come in many configurations. On monoplanes with one main strut, they may be just a single tube that connects that strut to the wing surface. On braced monoplanes with 'V' struts, they may brace both struts to the wing surface and also provide an interconnecting tube between the two jury struts to form a jury strut assembly.