During high school, I have learned that we are in the solar system and there are some planets orbiting the Sun.
Yes that is correct. According to current definitions of what counts as a planet, there are 8 planets orbiting our Sun $-$ Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are also a collection of smaller "Dwarf Planets" which include (in no particular order): Pluto, Ceres, Eris, and Makemake. There's also a lot of other minor solar system bodies out there like asteroids, comets, etc.
but there are stars, planets, constellations
Yes, there are many other stars out there besides our own (as you can see by simply looking up at the night sky). Each of those stars has the potential for having its own planets, much like ours does. Any planet outside our own solar system is referred to as an exoplanet (exo- meaning outside or external). Within the past 20 or so years we've developed the technology to find exoplanets around other stars and this TRAPPIST-1 discovery was simply the latest and most sensational discovery. To date we've discovered ~3500 exoplanets.
What confused me a lot are these two pictures. [...] And this definition of an exoplanet.
Those pictures, while seeming to show the the TRAPPIST-1 planets in our solar system, are not indicating that those planets actually reside in our solar system. Instead, what they're trying to do is compare the new TRAPPIST-1 system to our own so you can see the relative sizes. In your first image, you see Jupiter along with the orbits and distances of a few of its Moons. Next to it, you see the TRAPPIST-1 system with the seven planets we know of around it. Next to that is the Sun where you can just begin to see the orbit of Mercury (with the other planets beyond the bounds of the image. The point here is just to compare sizes of orbits. Clearly this TRAPPIST-1 system is much smaller than our own since all 7 planets orbit closer to the main star that even Mercury does for us. However, it is still bigger than Jupiter and its moons. They're not trying to make the point that these systems reside in the same place, just putting them together for comparison.
As for the second image you have, what they're trying to show here (I assume) is the habitability zone. Around every star, depending on the star's size and temperature, there will be a good distance to be from the star that is optimal for life to exist. You don't want your planet to be too hot (by being too close) or too cold (by being too far). This image appears to be showing you the regions where a planet can be at a good distance from TRAPPIST-1 and still be at a good temperature. There is the Inner B line which I take to be the Inner Boundary of this zone. In other words, if a planet is any closer than this, it will be too hot. There's also the Outer B (or Outer Boundary) where beyond that, any planet would be too cold for life. Finally they have the Earth B. I can't be certain but I believe this point is actually the distance for the "optimal" temperature that we have here on Earth. In other words, the best chance a planet has of being at a good temperature for life is to orbit between Inner B and Outer B and ideally be at the distance of Earth B. This image also shows that the orbit of the planet TRAPPIST-1e is close to Earth B and still inside Outer B, meaning it is likely to be at a hospitable temperature for life (which is good news for us!). I won't go into what an AU (aside from mentioning it is a unit of distance) or the frost line, since they're not pertinent to the point of the image. Feel free to ask other questions for that.
Lastly, the definition of an exoplanet you found describes it an "extrasolar planet". That just means it is beyond our solar system $-$ "extra-" meaning "outside" or "beyond" and "solar" referring to our own Sun.