What do yogis believe?

A yogi is someone who practices yoga, usually in its fuller meaning:

[A yogi is] one who is bound by a code of moral conduct and restraint (including celibacy) with a view to the realization of moksha (liberation)... throughout the East, the Words are often used to describe Buddhist monks or any lay person who is devoted to meditation. ~ Wikipedia

The title of "yogi" is usually reserved for someone who is an advanced practitioner and has achieved a high level of spiritual insight.