The external part of the ear (pinna) helps you to hear sounds arriving from the front and sides of you by shielding some of the sounds arriving from the back of you. This can be helpful when listening to speech in the presence of background noise. However, the location of the microphone on some hearing aid styles, in particular receiver-in-canal (RIC) and behind-the-ear (BTE) hearing aids, are not shielded by the external ear.
Hearing aids with two (dual) microphones are able detect the directions of sounds based on the principle that sounds arriving from behind with reach the back microphone before the front one, and vice-versa. As a result, the hearing aid can then focus on speech and away from noise (if they are arriving from different directions).
Fixed directionality is when the hearing aid only picks up all sounds directly in front of you. It will only help when noise is directly behind you and speech is directly in front of you.
Adaptive directionality is when the hearing aid continuously monitors the listening environment in search of speech before focusing in that direction and away from noise (again if they are arriving from different direction).
Narrow directionality compared to fixed and adaptive directionality, narrow directionality projects a more narrow beam to provide additional focus on speech.