Why is mining a big thing to make bitcoins? Don't understand what is going on here.
I don't understand it 100% either but here's my understanding:
There is some sort of publicly available numeric code at any particular time which describes all of the previous BitCoin transactions somehow. There is a function that can be applied to this code to create a new code but it requires you to guess a starting number from a HUGE range of possibilities. If your starting number and the function act on the code in such a way as to create nearly 0 "remainder" to some degree of "closeness" to 0 (e.g. 0.00234 is closer to 0 than 0.01112 because it has three leading zeroes instead of two).
Once somebody has found a solution to the required level of closeness, his solution becomes the NEW numeric code for the next guy to try to calculate an answer to, and he is rewarded with BitCoins - fewer of them as the answers get more complex until all the BitCoins have been "mined".
This process somehow provides protection against hackers and cheats and is necessary. Once the last BitCoins have been mined people will be hired to do this job as a normal job, being paid with currency that already exists, but for now, there are BitCoins "held in reserve" to pay these people with.
In order to guess lots of initial numbers and process the equation to find answers in statistically large enough amounts so as to actually solve the problem you need computers to basically guess numbers randomly until they get the answer you need. It costs money and energy to run these computers and only the person who gets the right answer gets paid and everyone else has to start over. This happens about every 10 minutes. The more computers you have running and the stronger their computing power, the more likely you are to get the correct answer first.