A fetish is sexual excitement in response to an object or body part that’s not typically sexual, and if you find out that your partner has one that doesnt make sense to you, there are ways to behave to not hurt their feelings.
Note that fetish is more common in men than women and as explained above, it is the 'over-sexualisation' of things that would normally not draw sexual excitement from anyone. It is not uncommon to hear that people have fetishes for shoes, feet, a particular colour etc.