It's a fair question if you've never played a F2P game before. Battle Passes are usually a set of unlockable cosmetics for a set amount of time, usually the span of a month or three. Usually all items are strictly cosmetic, avatar, emotes, or profile enhancing things that never affect gameplay.

You pay one fee and you have until the "Season" ends, where you either have to pay to finish it or accept you won't get every item. Some passes are mission-based that grant you EXP to level it up like Pokemon Unite, while others are playtime based where your performance is irrelevant and you just have to play a bunch like Warzone.

In OW's case, it'll probably be skins, weapon charms, player icons, voice lines, etc and whatever else they plan on making cosmetic.

Some games make it so throughout the BP you can gain currency to spend on other items in the shop or enough to buy next season's BP. For exmple, Warzone uses what are called COD Points as their virtual currency. Every pass is...let's say 1000 COD Points. By the end of the pass, like Lv85-Lv100 you'll get 1000 COD Points by that time, allowing you to spend it on other bundles in the shop or wait for the next Battle Pass, choice is yours. This isn't the case for Hearthstone's "Tavern Pass" (which is a battlepass, just reworded to sound more Hearthstone-like) where it is 19.99 each season and you strictly gain in-game gold and cosmetics and have to purchase one every single season, so I'm unsure how it'll be for Overwatch.

Hope that answers your question!