Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? What accord has Christ with Belial? Or what portion does a believer share with an unbeliever? (2 Corinthians 6:14-15)
A yoke was a wooden beam that kept two oxen together when ploughing.
Here is a photograph of two oxen yoked together which I took while travelling in India a few years ago.
The verse seems to be quite general — any sort of binding agreement between believers and unbelievers is to be avoided. I would think marriage is the primary example, but the verse has also been interpreted as a warning against a believer and an unbeliever going into business partnership together. There are many verses that say that a believer should only marry a believer. e.g., 1 Cor 7:39; Deut 7:3-4; 1 Kings 11:2.
While a believer and an unbeliever can still love each other, they do not share the same major goal in life. So the most important thing in the life of the believer can never be shared with his/her partner. Also, when children come along, there will be inevitable differences about how they should be raised and what they should be taught. Consequently, God has instructed us to marry fellow believers. I think it would be wrong to ignore this instruction.