If you are throwing a party, you will usually only invite the people you like and know well. Others are excluded and not invited. In daily life we distinguish between people all the time, based on age, gender, background, you name it. We do not always treat everyone equally. Because everyone is not the same.
So, it is normal to distinguish between people and to treat them unequally. In most cases, no one will care. But in some situations, it is not okay and even forbidden.
An example. Refusing to hire someone as a dentist’s assistant because they do not have the right diploma is not discrimination. To do the job properly, you need a diploma. Everyone understands that.
But rejecting someone because of his or her background, skin colour, or sexual orientation is discrimination. Because it means making a distinction based on a personal characteristic that is irrelevant to the job of dentist’s assistant. This form of unequal treatment is prohibited - or should be.
You can make a distinction between people - also based on gender or skin color. It's not forbidden to have a preference for a man or a woman, for someone with a specific skin color or origin. It is just not allowed in situations where that distinction should actually not play a role.