The Washington State School Directors' Association is one of many organizations that publishes regular legislative updates for its members. Here they provide a handy description of what's happening these days.
...Remember that in order to become law, a bill must be adopted by both the House and Senate in the exact same form and then be signed by the governor. If the opposite house adopts an amended bill, it must be returned to the original house for “concurrence.” If the original house agrees with opposite house amendments, they will “move to concur” and vote on final passage. If the original house disagrees with opposite house amendments, they will “move to not concur” and send the bill back to the opposite house. Once back there, the opposite house can strip off the amendments; negotiate new language and adopt new amendments; or “move to insist on its position. Bills can ping-pong back and forth between the two houses until agreements are reached — or until it is determined that there will be no agreement. At this point, a Conference Committee can be appointed to formally negotiate a compromise... In order for a Conference Report” to be adopted, at least two members of each house must agree to the new bill... they are free to amend the bill under consideration in any manner they see fit — including throwing the entire bill out and rewriting the entire bill, so long as it remains consistent with the "scope and object” of the original bill (that is, it deals with the same subject).
There you have it. Everything you ever wanted to know about the concurrence process.
The legislative website also provides an overview of concurrence and other legislative terms.
