In eukaryotic cells, the RNA produced directly from a protein-coding gene is usually a pre-mRNA rather than a translation-ready message. The transcript is processed in the nucleus to form mature messenger RNA (mRNA).

Three major processing operations are especially important.

5′ capping

Soon after transcription begins, the $5'$ end receives a modified guanine-containing 5′ cap. The cap protects the RNA from degradation and helps proteins recognize the RNA for export and later translation.

3′ cleavage and polyadenylation

The transcript is cleaved at its $3'$ end and a stretch of adenine residues called the poly(A) tail is added. The tail contributes to RNA stability, export and translation efficiency.

The poly(A) tail is added after transcription; it is not simply copied from a long run of thymine bases in the DNA template.

RNA splicing

Many eukaryotic protein-coding genes contain introns, transcript regions that are removed from the pre-mRNA, separated by exons, regions retained in the mature RNA.

A large RNA-protein complex called the spliceosome recognizes splice sites, removes an intron and joins the neighboring exons.

Schematically:

pre-mRNA:   exon 1 — intron — exon 2 — intron — exon 3
                         ↓ splicing
mature mRNA: exon 1 — exon 2 — exon 3

Exon does not simply mean 'protein-coding sequence': mature mRNAs also contain untranslated regions, and these can be part of exons.

After capping, cleavage/polyadenylation and appropriate splicing, mature mRNA can be exported from the nucleus to the cytoplasm for translation.

RNA processing therefore changes the physical RNA molecule produced by transcription without changing the DNA sequence from which it was transcribed.