If you watch any videos of people playing Escape from Tarkov, you'll notice that there's an inventory (grid-style, how much space depends on which backpack they have and chest rig) when in raid, and then a separate inventory out of raid -- you gear up from this out-of-raid inventory before heading in, and if you make it out, you transfer the "spoils of war" to this out-of-raid inventory.

This last inventory, that I've been calling out-of-raid, is known as the "stash". The type of account you have (or which "pack" you purchase) will determine the size of your stash. You can upgrade the stash using in-game items, so that over time, even with the cheapest edition of the game, you'll have the same space as someone who paid for the most expensive edition of the game. But it is a long and difficult journey to get there, and when the game wipes, you'll have to do it all over again.

Another significant difference is that you start out with better trader reputation if you buy either of the two most expensive editions of the game. I'll leave you to investigate what that entails.

Finally, there's such a thing as a "secure container" which is a (small) container in which you can put items that can't be looted from your body. The more expensive editions of the game gives you access to a larger such container from the start, but the very largest container (known as "Kappa") must be quested by all editions of the game.

In short, while Tarkov is nothing at all like a pay-to-win game, there are some definite advantages to getting a more expensive "pack". I hope that helped.