Oregon Senate Republicans are currently out of state. What is a state legislative walkout?

On June 20, 2019, the 11-member Republican minority of the Oregon State Senate did not come to a scheduled legislative session to discuss HB2020, a cap-and-trade bill. With only 18 Democratic members, the chamber fell short of the 20 members needed to take official action on the bill. Although Senate President Peter Courtney announced on June 25 that HB2020 did not have the votes necessary to pass, Republicans have remained out of state away from the legislature.
This story is one of the latest noteworthy state legislative walkouts, which occur when members of a state legislature leave the state capitol or the state entirely to prevent the passage of legislation.
How does this happen?
State legislatures require that a specific number of members be present to conduct official business, such as debating or voting on legislation. The minimum number of members required to conduct official business is known as quorum. The Oregon State Senate, for example, requires two-thirds of its 30 members be present to reach quorum.
Quorum requirements are often laid out in state constitutions. Forty-five states require a majority of legislators be present for quorum. Four states require two-thirds of legislators be present for quorum. Massachusetts requires two-fifths of state senators or three-eighths of state representatives to be present for quorum. In many states, there are also statutory requirements for quorum if a bill involves taxes or state finances.
There were two noteworthy Democratic state legislative walkouts in 2011. Democrats in the minority of the Indiana House of Representatives and Wisconsin State Senate left their states while the chambers were considering right-to-work legislation.