Bitcoin, at its core is a number, albeit a long, complex number. In order to obtain that number you must work an equally complex mathematical equation, which is called mining.
The Bitcoin algorithm was designed to make that mining process increasing more difficult as more blocks are added to the blockchain.
Your PC takes information from each transfer in the block. Parts of each user's public key, signed by the proper private key, are all used to create this beautiful piece of math. A new piece of information is then added to the
equation which pays you the block reward. Once the equation is assembled it is then "hashed" (or encrypted). Then, it is compared to the existing blockchain, thereby verifying the block, giving you the reward for it, and broadcasting it to the
blockchain making it historical fact. If your PC or mining equipment
completes this process first you get the block reward. As long as your equipment can perform this process billions of times in succession you have a good chance of obtaining that
As most people do not own equipment capable of reaching such lofty goals another option would be to join a mining pool. Groups of miners work together to find the solution that reaches the reward; which is then split between the participants.
Since the process involves comparing every piece of information (coin) being transferred in the current block, with the existing blockchain (to avoid double spending and fraud) it is only logical that, with every block, the mining process becomes more difficult since there is more to be compared.
So, the difficulty in mining is one that comes from tracking each Bitcoin transfer, with every transfer in every block before it, since the history of the coin.