Xenophobia (from Greek "Xenos" foreign, strange + phobia) is a diffuse and unclear concept, but in practice the word is often used with the aim of associating criticisms of mass immigration with mental illness. It can be seen as a form of attempted guilt by association and name calling.
The politically correct sources using the word often state that it is distinct from racism, by focusing more on foreign/strange culture than on race. Therefore, the word can be used as a smear on critics of mass immigration who state that they are race denialists and therefore not racists.
However, this distinction is often ignored, with racism often stated to be a form of xenophobia and race denialist mass immigration critics often labelled as racists despite their race denialism.
Being more cautious of unknown strangers than of known friends is likely not a form of mental illness.
The politically correct sources using the word, and related "-phobia" word, apply a double standard and do not use "-phobia" words as labels for criticisms of Whites, heterosexuality, Christianity, and so on.