AD stands for Anno Domini, Latin for, "in the year of our Lord". AD refers to all years after year 0. Whilst I'm writing this, it's 2023 AD.
On the other hand, BC stands for, "Before the birth of Christ". This refers to all years before year 0, such as 2023 BC.
CE and BCE are also the same as AD and BC, respectively. CE and BCE are used in non-religious settings.
Hope this helps!