In addition to randir's explanation of honorific usage of the tagalog term Kuya for male relatives and non-relatives, Kuya can also be used for a younger male in certain circumstances. In this case, the speaker's older blood relative or close friend has a male significant other younger than the speaker him or herself. The relative's or friend's male partner would be called Kuya regardless of age, especially if he is a husband.
While randir indicates that Kuya can be "used as a term of affection to an older male who is not related to the speaker" some females use it for duplicitous purposes. This may be to warn a fellow female friend or relative that a male in question is taken. For instance, a female speaker's older female cousin may tell her to, "Go show Kuya Jon-Jon the bathroom", as a sly way of indicating that he's not available. Another example of underhanded usage is if an older male friend not already called Kuya apparently has unwelcome amorous intentions, the female might call him Kuya to give him a hint that she is not interested in him in that way. To be called Kuya implies affection and respect but may also be an annoyance.