A limited bundle is a bundle that can only be purchased a certain number of times before it runs out and becomes unavailable. When you click on the "Buy" button on the purchase form, a bundle will be reserved for you so you can complete payment. If you don't complete payment, the reserved bundle will be returned to the pool of available bundles after a period of time. Once a limited bundle runs out, a few bundles may return due to expired orders or refunds, but would otherwise be unavailable. Limited bundles may or may not receive a limit increase depending on the type of contents.
Similarly, a limited tier is a tier that is only available in limited amounts. If you purchase a limited tier, all tiers below it, including limited ones, will be available to you.
If you do not intend on completing a limited bundle/tier purchase, please cancel the order from your purchases page so others may grab it.