The big blind is a forced bet that must be posted before the hand even starts by the second player to the left of the dealer.
Along with the small blind, big blind creates an incentive for players to play their hands since there won’t be any reason to enter the pot unless you have pocket aces without it. While the big blind is considered the worst position at the poker table, learning how to approach it to minimize your losses and protect your EV is essential.
If you are playing tournaments, the blinds will constantly increase on predetermined time intervals, so the big blind will increase as well as you progress in the event. On the other hand, cash game shave a fixed big blind that is not going to change while you stay at a particular level. For example, if you are playing a $1/$2 cash game, the big blind will always be $2.