Context:

This quote is from Chapter 33 of George’s most important and well-known work, Progress and Poverty

The important thing to note here is that Henry George was using the classical definition of a monopoly, not the sense of a single seller dominating a market as we know it today, but controlling any asset where new production and entry into the market is impossible (entry-monopolies as Georgist economist Fred Foldvary calls them). The most important of these is nature (which he grouped under the umbrella term of land), and which he describes as the following:

Rent, in short, is the price of monopoly. It arises from individual ownership of the natural elements — which human exertion can neither produce nor increase.

(Rent here means economic rent, the returns to natural resources).

Because monopoly is a complex term for this, I’ll be using finite resource to describe this from now on. But it’s for this reason that, in a weird sense, we actually tax production twice. Currently to get public revenue we levy taxes on workers, businesses, and sales. At the same time we allow people to withhold finite resources like land and use them to extract from those who produce; inviting those who further want to profiteer off withholding finite resources, which only drives costs up and worsens the whole situation.

In order to remediate this, the Georgist solution is to simply untax the things people make, and instead compensate (or mitigate) the finite resources people take. This includes compensating all natural resources when they’re taken (and tangentially when they’re destroyed through pollution), compensating/mitigating exclusive legal monopolies (like the ones given out by patents/copyrights over particular innovations), dealing with natural monopolies in some way so they can serve the public and not be extractive, and just dealing with any finite resources in general that nobody else can compete with in the economy.