Can someone pls explain me that concept when they say like : oh the level at a 25nl is way higher than at a 5nl! If you are good at 5nl, what would be the differences at a higher level like 25nl ? And 200Nl vs 25nl? what makes you see that the level at a table is very high ? At 25nl vs5nl or any other level, you'll still play tight with your good hands IP, 3bet with good hands, and adjust your play based on other players profiles.
Assuming you're talking about differences in online stakes, which is a bit of a derail from the original subject of this thread, but...
There is a definite difference between stakes, but the skill gap narrows as you move up higher and higher. At some point it's less about skill than about BR and comfort level at the stakes being played.
The winning style at 5NL online is very different from the winning style of 25NL. From 25NL to 200NL the difference is not nearly as large, but there still is one. 200NL regs are generally going to be more accomplished than 25NL regs, so adjustment is necessary. There will still be fish with money at most any stakes, but the reg to fish ratio tends to increase quite a bit as you move up.
Playing a simple TAG style at the micros is recommended for lots of reasons. It's lower variance, for one. People don't fold to your plays as much, so fold equity doesn't have the same value. Unsophisticated opponents don't respond as you expect to sophisticated plays. It's just much more of a gamble to play anything but a tight ABC game in order to protect your meager profits.
At higher stakes, regs are more aware of tendencies. If you play too TAG, they will exploit you and severely constrain your profits. You have to loosen up and play a more sophisticated game, especially post-flop, to realize gains. However it takes skill to manage that aggression properly, look for the right spots, adjust to different players, etc.
That is why we say it takes a different level of play or skill to advance in stakes. A major of part of successful poker is adjusting to the game, and success at the higher stakes require significant adjustment on an ongoing basis compared to micros where you can just grind a mostly static style.