together with unbelievers", means,

Do not develop your closest relationshis with people who are not believers. (This would include dating, marriage, best friends, business partners, etc.
(The word 'yoked' used in this verse comes from a Greek word meaning: a chain, a link, a bond, a connection.)

Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? 2 Corinthians 6:14

In the olden days of America, and still today in undeveloped countries, two oxen or other beasts of burden, were 'yoked' tegether, at the necks, with a large, heavy wooden yoke, to form a team capable of working together to pull a plow or haul logs, etc. These oxen had to be 'equally yoked'; in other words, be of the same size, strength, and ability. If a large, strong ox were yoked with a small weak ox, the team would work poorly together, with the weaker ox being unable wo pull its load, perhaps tripping or even falling, thereby causing he stronger oc to stumble or fall as well. In just this way, the very close or intimate relationship between a believer and unbeliever is a dangerous and unworkable one.

Can you think of any examples of how being "unequally yoked with unbelievers" could cause us to stumble and fall from our Christian walk?