What is Block in Bitcoin?
In Simple Words, its something to hold bitcoin's network transactions. More preciesly those transaction are waiting to be processed. Those blocks have bundles of transaction that miners perform mining on. If a miners mines a block, transaction gets completed. Also the payment finishes processing.
At before blocks were limited to 36MB transaction data. From 2010 it was decreased to 1MB. Thus it takes lesser time now to mine blocks.