Definition of High-roller
A player who qualifies as a high roller is a player who plays high stakes and high roller tournaments. Not everyone will agree on the precise definition of limits, but here's a rough breakdown of how poker limits are categorized. If you play tournaments at 20??? and under, you are playing micro-limits. If you play tournaments between 21??? and 100???, you are playing small limits. If you play tournaments between 101??? and 600???, you play medium limits. And if you play tournaments between 600??? and 10,000???, you are probably playing high stakes. Tournaments of 100,000??? and above are often referred to as nosebleeds. In cash games, tables that will have blinds at 5???/10?? (1000??? buy-in) are often referred to as high stakes tables. A high roller is a player who plays these limits.
High-roller is also synonymous with a very good player
The higher the limit, the stronger the players. At least the ones who win. When we talk about a high-roller player, we are also talking about a very strong player, among the world's elite poker players. Some tournaments are also referred to as high-roller tournaments. Again, there is no precise indication of the accuracy of the term, but let's say the highest buy-in tournaments in a series of tournaments are called high-roller tournaments.