In writing academically or speaking formally, it is important to choose your words in a way that you do not distinguish between genders or include both genders equally.
Nowadays, some people might find the gender-discrimination very offensive.

Singular 'they' is used instead of 'he' or 'she' to refer to a person whose sex is not mentioned or is unknown.
People who do not identify themselves as either male or female (non-binary people) usually prefer to be referred to as they.

That is why, in modern English, they have created a new pronoun called the singular 'they'. In the past, 'he' was used to refer to both men and women.

Some people prefer to use he or she instead of 'they' in speech and writing:

Another alternative for avoiding gender specificity is writing He/she or (s)he. But all these variations are a bit frowned upon. And your best choice is to use 'they'.

It is better to try to change the sentence, using the singular 'they'.

With 'they' you can also refer to people in general, despite their gender.

When 'they' is the subject of a sentence, whether it is the singular 'they' or plural 'they', you must use plural verbs. Never say or write 'they is'. Always use 'are' with this pronoun.