The statement is quite nebulous (hey, its coming from a politician), but Boris Johnson isn't the first government politician to float the figure:
Because we value the NHS so much, the new £20.5 billion funding settlement announced by the Prime Minister in June provides the NHS with funding growth of 3.4% a year in real terms over the next five years. This means the NHS’s budget will increase in cash terms by £33.9 billion, rising from £115 billion this year to £121 billion next year, £127 billion in 2020-21, £133 billion in 2021-22, £140 billion in 2022-23 and £148 billion in 2023-24.
Baroness Manzoor - 7th January 2019
The £1.8 billion funding is in addition to the extra £33.9 billion, in cash terms, the NHS is set to receive every year by 2023/24 through the Long Term Plan agreed last year. Over £1 billion of this will be spent this year, meaning an annual increase in the NHS’s capital budget of 30%.
Published 5 August 2019 - gov.uk (This would have been under Boris Johnson as PM, but I am leaving it here to indicate that the recent speech was not his governments first use of the amount).
In essence, its a cumulative increase in spending over the period, with the increases year on year adding up to the £33.9Billion figure.