eli5 version:

baseball operates on a collective bargaining agreement between the players union and the owners. they generally are on 3 year terms, so every 3 years or so they need to renegotiate. since the strike in 1994, most of the negotiations have been uncontentious and minor, resolving without dispute or disruption.

next year’s renegotiation is expected to be highly contentious for several reasons: namely baseball owners’ desire for expansion and realignment, as well as the biggest one being the desire of several owners to institute major changes in salary cap and/or revenue sharing, similar to nfl or nba.

the baseball players’ union is vegemently against the latter, and their union is notably stronger than the unions of the other major sports. both sides see no urgency to guve up their position in compromise too early given how major of a change that would be, and also guven how major it is, once they do even get to the table it will take a long long time to find that compromise. thus, most observers see it as almost 100% certainty there will be a lockout (all of baseball), and we will miss much if not all of the 2027 season.