BC is denoted for the word ‘Before Christ’. The word refers to the time period before the birth of the Christ (Jesus). Therefore the years before the birth of Christ are considered as ‘BC’.
AD is acronym of the word ‘Anno Domini’. Anno Domini is a latin word which means that ‘In the year of Christ (Jesus)’. Therefore the time period which occurred after the birth of the Christ is considered as ‘Anno Domini years (AD)’.
For better understanding of time calculation let us discuss with a timeline shown below
image source: https://quatr.us/science/what-does-a-d-mean.htm
As shown in the timeline, the years before the birth of Jesus are counted in a reverse direction (i.e 300 BC←200 BC←100 BC). Hence if the years are counted in reverse direction the first year which comes is the greater number then the next one. For better understanding, let us take an example: Buddhism started in 563 BC and the war of Mahabharata ends in 3121 BC. Now from the above given events which event happened first? The correct answer is the war of Mahabharata that ends in 3121 BC. Though 3121 BC is greater number then 563 BC but as it is counted in reverse direction. This can be considered just like the negative X-axis in a Cartesian coordinate system.
As shown in the timeline Anno Domini years starts after the birth of Christ, so the years counted is in the increasing order from 100 AD→ 200 AD→ 300 AD→. Here the years will be mentioned in the forward direction in the timeline. For example which year will comes first from 27 AD and 300 AD in the history? The correct answer is 27 AD will come first in the timeline as per the chronology.
So we are currently living in the 2nd decade of the 21st century.