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Transcription is the process by which the information in DNA is copied into messenger RNA (mRNA) for protein production. Transcription begins with a bundle of factors assembling at the promoter sequence on the DNA (in red). Here, two transcription factors are already bound to the promoter. Other proteins arrive, carrying the enzyme RNA polymerase (in blue). To initiate transcription, these assembled proteins require contact with activator proteins that bind to specific sequences of DNA known as enhancer regions. Once the contact is made, the RNA polymerase races along the DNA to transcribe the gene.
source: https://www.dnalc.org/resources/3d/12-transcription-basic.html (this link also has a nice video which explains the subject)
Simply saying Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (mRNA) by the enzyme RNA polymerase