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
How typical of John to complain about something as if it is a hardship to him when really it is the opposite!