This idea of enclosing the common lands is mentioned twice in this week's chapters---the reference you made and when the gentlemen were talking after dinner at John and Fanny's.

John spoke of it as being a 'drain' on his income that this enclosure was happening.

My Norton Critical edition makes this note:

During the 18th and especially later during htre Napoleonic Wars, common land--which labourers had previously been able to use for pasturage and for the gathering of wood for use as fuel--was increasingly enclosed in order to create large farms under the ownership of wealthy landowners. The irony here is that John Dashwood is complaining about a phenomenon that is (once again) impoverishing others and enriching heimself, for it was small farmers and labourers, rather than major landowners, who lost out as enclosure became more fequent