A Bet in poker tends to be interpreted as an ‘action’ play such as opening the betting in a hitherto uncontested pot.
However, other than folding, any option could be described as a bet, with a check, for example, essentially being a bet of zero.
The Big Blind, therefore, can be seen as a bet in that it is technically the first action/bet – albeit forced – that the rest of the players must at least match in order to remain involved in the hand.
Betting must be made in a specific sequence, with the convention being that it proceeds in a clockwise direction around the table. The Dealer Button moves one place to the left with each successive hand so that after an ‘orbit’ everyone at the table has had the same betting opportunities in terms of their position relative to the opposition.