Allow me to introduce you to Jack. Jack does it all. He swims on his college swim team, he sings in the men's choir, he plays the guitar, he paints artwork -- and that's just the tip of the iceberg. Jack double majors in math and business, and he still has time to study programming and web design in his spare time. This begs the question, "How does Jack manage to do all of that?" The answer: barely. Jack has never won a race in the pool. He sings out of tune 40% of the time. His grasp of the musical and visual arts is amateur at best. Jack is a consistent 'C' student and hasn't advanced past the basics of writing code for a website. Jack does a lot of things, but he doesn't do them well. Jack is one of the People You Don't Want to Be.
A Jack of All Trades is someone who is experienced in many areas. However, there is a subgroup of these tradesman. This subgroup contains those who dabble in many activities, but are not proficient in any of them. An individual in this subgroup is often referred to as a "Jack of All Trades, Master of None." Basically, these "Jacks" spend so much time jumping from activity to activity that they don't have sufficient time or energy to actually learn any of them, let alone master one.
There is no problem with wanting to be involved. On the contrary, a well-rounded person provides themself with many possible paths through life; they increase their chances of success. Almost all college applications inquire on your past activities and general involvement. It is a sort of measure of one's personality, albeit a weak one. A high level of involvement becomes a problem, however, when one cannot say they are exceptional at something. By nature, many of us are given talents; we are meant to be good at something. It may take some searching to find your talent, which may push you to the realm of Jack for a while, but once your talent is found, hold onto it. Learn it. Master it. Amateur status will get you through life. Hard work and mastery will allow you to excel throughout your life.
To avoid being a Master of None, you must know what you want. You must set goals and work to achieve them. Wandering aimlessly through life and attempting task after hopeless task will most likely end in disappointment. You don't want to be a Master of None.