That's a very good question. All Portuguese nouns have gender even if they refer to inanimate or abstract things. Different articles (the equivalent of "a/an/the" in English) must be chosen to match the gender of the noun. In fact, one of the few tips that Duolingo Portuguese currently offers is on this topic:
The fact that nouns have gender (and number, i.e. they can be singular or plural) adds a level of complication to Portuguese which is completely missing from English. For example, in English there is just one form of an adjective. In Portuguese there are usually four, one for each combination of masculine or feminine and singular or plural.
Duolingo teaches these things relatively slowly, so if you aren't already aware of them you can give yourself a head start by reading something beforehand; try looking at some of the resources I mention here: