What is indoctrination?
Indoctrination is brain washing. The term indoctrination implies long term and subtle methods.
Indoctrination means when you purposly educate women and men differently. For example when Hitler was in power he used indoctrination methods. He used to make women learn about cooking and childcare and men learn about maths and science. This was to enable men to go out and earn a living, and for women to stay at home and be housewives.
To get into any of the Ranger Battalions as lower enlisted (E4 and below), you simply have to pass the two week indoctrination course (used to be called RIP - Ranger Indoctrination Program... now they call it something else). For Corporals and higher, as well as officers, they must already have a Ranger tab.