Correct marginal rate (meaning)

In the Bpp textbook, it is written:

Employee contributions to occupational pension schemes get tax relief under ‘net pay arrangements’ where the employer deducts the employee’s gross pension contribution before applying pay as you earn (PAYE).

This gives the employee tax relief at the correct marginal rate (ie starting with the highest rate ) of tax without the need to make any additional claims.

I am sorry for asking such basic questions but I find the language used in the textbook a bit hard.

What is the meaning of “correct marginal rate?”

Another question: the employer, in his income tax computation, deducts employee’s gross pension contribution while computing taxable income, right?

But the employee doesn’t deduct this while working out the income tax computation….