What is meant by basal level transcription? .

In cells, some genes are constitutively transcribed. These genes are typically under damped feed forward and positive control mechanisms and their protein products code for basic metabolic machinery and other proteins necessary for cell life. If they weren't there, the cell would die; therefore, it is careful to always transcribe enough new mRNAs to replace those being degraded.

In cases when more of these products are needed - for example, when a cell faces a metabolic challenge - then transcription is upregulated from this basic or basal level. The cell meets its need; negative feedback at higher transcript concentrations or copy number reduces transcription of the gene or genes involved in the response, and their rate of transcription goes back to the basic level.

For a superb example, read up on hypoxia inducible factor and the cassette of genes it regulates.