Makhārij is the plural of makhraj. It literally means a place from which something exits. Technically it is an articulation point i.e. that place from which the sounds of the letters are articulated.
There are 17 makhārij (articulation points) according to Khalīl Ibn Aḥmad al-Farāhīdī:1
1) The empty space in the mouth and throat. The three lengthened letters (madd letters) are pronounced from here. Wāw sākinah preceded by a ḍammah, yāʾ sākinah preceded by a kasrah and alif.
The alif never takes a ḥarakah and is always preceded by a fatḥah
2) The lower throat. This is the part of the throat closest to the chest and furthest from the opening of the mouth. The hamzah ء and hāʾ ه are pronounced from here.
3) The middle throat. The ʿayn ع and ḥāʾ ح are pronounced from here.
4) The upper throat. This is that part of the throat which is closest to the mouth. The ghayn غ and khaa خare pronounced from here.