A roster is a specific set of players who represent a team in tournaments: five people in CS2, five in Dota 2, or another number depending on the discipline. When people say "team roster," they mean the actual people in the positions, not the organization's brand as a whole — an organization can exist for years, while the rosters within it change dozens of times.
The word is used in different contexts. "Main roster" refers to those who play in official matches. "Substitute roster" or "reserve roster" refers to backup players in case someone from the main lineup is injured, banned, or simply not performing. "To build a roster" means to sign the required number of players for specific roles — for example, finding a support, carry, and mid player for Dota 2, or a sniper and an IGL (the person who devises tactics and calls the shots during a round) for CS2.
How it works in practice
An organization is a legal entity, a brand, management, sponsors. A roster is what physically plays under that brand. That's exactly why news often separates these two things: "the organization signed a deal with an investor" and "the team announced a new roster" — these are two different events. The first is about business and money, the second is about who exactly will be pressing the buttons in the match.
Changes to a roster are described differently depending on scale: swapping one player is called a transfer or a substitution, while disbanding the entire team is called a disband. Between these extremes there are partial rebuilds, when an organization keeps the core but changes one or two positions to fit new tactics or because of internal conflicts within the team.
An example from current events: an organization's CEO might publicly talk about wanting to build a roster for a discipline new to them — this highlights that a roster doesn't appear automatically just because there's interest in the game. It requires specific players, a coaching staff, bootcamps (joint training camps for practicing tactics), and a budget that varies from discipline to discipline. That's why organizations often first test their strength in one game, postponing the decision to enter another until they understand how much a truly ready, competitive roster would actually cost.