Context:

The symbol is the Single Tax campaign button from the 1890s, back when Georgism was at its peak of popularity.

But simply put, Georgism is broadly an ideology that argues that the value of production, labor or capital, should belong to producers, but the value of monopoly power in controlling finite natural resources, exclusive privileges, natural monopolies, or the like should belong to society or be dismantled (e.g. land, patents/copyrights [Georgists generally support keeping rewards for innovation], utilities, etc.). It might be weird to think of monopoly power coming from owning natural resources like land, but the classical economists like George argued for it from the principle of barriers of entry; nobody can make more of nature to come into the market and compete with incumbent owners.

In practice, Georgists would support eliminating taxes on work and investment, so no income taxes, payroll taxes, sales taxes, capital gains taxes, taxes on buildings, etc. and instead taxing land, which is the largest source of economic rent, and other finite natural resources; taxing/eliminating exclusive legal privileges (like Harberger taxes on IP or prizes instead of patents); as well as having natural monopolies be beholden to the public (be it a variety of reforms like direct ownership or auctioning off the rights to a natural monopoly to get the full monopoly cost).

This is all to say, Georgism brings the market economy and anti-monopolism to its logical peak by opposing private rent-seeking from monopoly power while supporting actually productive labor and capital going untaxed and unburdened to produce and provide for the benefit of others.