The two terms are actually interchangeable. "Split" just indicates a bird that is known to carry one copy of a genetic recessive trait. Since that bird is only carrying one copy of the recessive trait, they are "split", or heterozygous, for the hidden trait.
So, picture a black rooster with a hidden white allele (one copy of the gene). He looks black, he acts black, therefore he is black. But he has a surprise hiding inside.
Now picture a black hen with that same single copy (an allele) of that white gene. She looks black, she acts, black, but she also is carrying a hidden surprise.
You breed that rooster to that hen. You get 100 chicks from that match. Statistically, you would get 25 chicks that are pure for black - they do not have that hidden gene inside; 25 chicks where the white allele from the sire met up with the white allele from the dam and created the surprise white chicks; and you would get 50 chicks that are each carrying that hidden single copy of the white gene.
Remember that a true recessive trait must be in pairs to show up in the chicken, and remember that a chicken gets one copy of each gene from each parent.
Does that help? :)