The distinction of gender into masculine (مذكر) and feminine (مؤنث) is an important feature of Arabic, unlike English where grammatically the great majority of words do not make this distinction clear, e.g. in English ‘student’ does not imply the gender of the person it refers to, while in Arabic (طالب) is masculine while (طالبة) is feminine.
In Arabic, all nouns must have a gender whether they refer to animate or animate objects, e.g. the word for chair (كرسي) is masculine while the word for table (طاولة) is feminine. This grammatical gender is totally arbitrary, and a good example of its arbitrariness can be exemplified by the fact that we can have 2 words that refer to the same object and carry different genders, e.g. there are 2 words that mean window (شباك) which is masculine and (نافذة) which is feminine. Learners of Arabic should learn the gender of all nouns as they meet them.
Gender is not only indicated in the noun, it must also be indicated in all grammatical elements that accompany them such as adjectives, relative and personal pronouns, demonstratives, etc. Consider the two sentences that follow:
“This is the new engineer who works in the Kuwaiti company.”
هذا هو المهندس الجديد الذي يعمل في الشركة الكويتية.
هذه هي المهندسة الجديدة التي تعمل في الشركة الكويتية.
Unfortunately, most Arabic dictionaries do not indicate the gender of words, which can be very frustrating to learners of Arabic. However, if they provide examples, the use of adjectives and other grammatical elements can give an indication. If you are confused about the gender of a certain word in Arabic, try to google it, and a quick look at the contexts in which it is used, can indicate the gender by looking at adjectives or other grammatical elements that are used with it.