Single intent visas are non-immigrant visas that do not allow immigrant intent while dual intent visas do.
For example, someone applying for a B-2 or F-1 visa must demonstrate that they have ties to their home country and will return there at the end of their stay in the US. Someone applying for an H-1B visa does not need to meet those requirements as it is a dual intent visa. K-1 visas are a bit unusual because they are technically non-immigrant visas but their sole purpose is for immigrating to the US, so they are technically dual intent.