This doesn't really have much significance for law, but come on, X was a very VERY difficult letter where no words actually fit. So.
Xenophobia is a fear or hatred towards foreigners, foreign countries, or anything foreign.
The significance this has with law is based on the European Union.
Xenophobia appeared in Article 67 of the TFEU and states that "The Union shall endeavour to ensure a high level of security through measures to prevent and combat crime, racism and xenophobia". As a result of this aim, a regulation (which is directly applicable in all member states, meaning that it does not need to be transposed into law and cannot be disguised as anything else) was created to deal with this.
The regulation establishes a European Monitoring Centre on Racism and Xenophobia with a view to providing the community and its Member States with objective, reliable and comparable data at European level on Racism, xenophobia and anti-Semitism in order to help them when they took measures or formulated courses of action within their sphere of competence.