A Battle Bundle includes a Battle Pass for that season, as well as 25 Tiers for less than what you would pay to purchase them separately.
A Battle Bundle lets you get a big head start and reach max level a lot faster.
You can buy the Battle Bundle at the beginning of the season, instead of buying just the Battle Pass. It is part of the Battle Pass Offers offered to you.
The Battle Bundle was first offered during Season 3. The Season 3 Battle Bundle had a cost of 2,800 Vbucks instead of 4,700.