Two things of note here.

1) President Trump says mail-in voting in Nevada would increase turnout so much that it would make it difficult for GOP candidates to win statewide races. https://twitter.com/realDonaldTrump/status/1290250416278532096

2) Democrats have pressed President Trump to back more money to help states get ready for more mail-in ballots, and to help the Post Office deal with the surge.

The White House opposes that extra funding.

Nevada had a June primary with expanded mail-in voting. That was approved by the GOP Secretary of State. This new law would allow the Dem Governor to authorize such a plan for the November elections. Registered voters would get a ballot sent to them.

The GOP Secretary of State says the June 2020 Nevada mail-in primary (which also had limited polling place options) did not generate fraud.