It could mean a lot of means. Could mean that you fought everyone and none were able to beat you, or it could mean that you carefully selected opponents to meet your stylistic requirements, or it could also mean you're still too green and yet to step up.
depends... You can't boast about it when your opposition isn't where it should be at the time it should have been.
every fighter in the history of combat sports CAN be beaten. undefeated means no one had been able to beat said fighter. take Floyd, he was truly unbeatable from 130 to 140. 147 he was pretty unbeatable until the last few years, even Floyd is very beatable right now and if he is beatable everyone is beatable
Being undefeated isn't really the end all be all honestly the only unbeaten fighter that will rank real high in history is Mayweather simply because of the resume and accomplishments that coincide with it. It's about who you fight, when you fight them, and what level they were at when the fights happened.
Ali, Sugar Ray Robinson, Joe Louis all ATG's but neither was undefeated. The only undefeated ATG I know was Rocky Marciano. To be undefeated like Mayweather & Calzaghe all that means is you've cherry picked your opponents quite well but have also been given decisions over fighters that really beat you but got shafted. Real ATG's fight anyone anytime anywhere