Messenger RNP (messenger ribonucleoprotein) is mRNA with bound proteins. mRNA does not exist in vivo "naked", mRNA is always bound by many different proteins while being synthesized, spliced, exported and while being translated in the cytoplasm.
When mRNA is being synthesized by RNA polymerase, this nascent mRNA is already bound by RNA 5′ end 7-methyl-guanosine capping enzymes. Later mRNA is bound by exon and intron definition complexes, and splicing snRNPs. The spliced mRNA is bound the other set of proteins which help in export from the nucleus to the cytoplasm. In vertebrates exon-exon junction are marked by exon junction complexes which in the cytosol could trigger nonsense mediated decay if the exon-exon junction is downstream of the stop codon.