A non-cumulative tax code is represented by the X at the end of the tax code. This means that tax is calculated on the gross pay earned in the current pay period only. Whereas, a cumulative tax code will assess an employee’s total gross pay for the tax year against the current point in the tax year to calculate the tax due.
Helen's answer is about tax codes. The actual figure against 'cumulative tax' on your pay slip should be the total amount of tax that you've paid on your earnings in the current tax year. (i.e. from 6th April).
Why do I always forget that Piggy is in Spain? (Thanks, Hopkirk!).
The Agencia Tributaria bases its tax years on calendar years (i.e. commencing on 1 January), so your 'cumulative tax' should be the total amount of tax that you've paid so far this year.