You will find information and explanations for the grammar taught in the lessons in the Tips and notes section.
To answer your question, there are three grammatical genders in Greek: masculine, feminine and neuter. Each noun has its grammatical gender and you just learn it when you learn the word, you can't guess it because it's not describing a quality of what the word says. For example, the words for boys and girls in Greek are neuter, not masculine and feminine.
It seems hard if you try to do it all at once. It's best to do each exercise using the Tips & notes as mentioned above by D_.. and also use the Drop Down Hints frequently so you get used to each word and see it used in the sentence.
See here for other helpful links.