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It means, you can push twice as much bandwidth toward some path then it can carry.
For example, imagine a 21 port switch, which has 20 FE copper ports and one gig fiber port. The 20 FE ports could, in theory, accept twice as much data then the gig port could upload. So, in that situation, the gig port would be 2:1 oversubscribed.
When reading device specifications, you'll often see how much bandwidth the switch's fabric can handle. Like the prior example, all the switch's ports might be able to accept more bandwidth than the fabric can pass.
Today, many switch's have fabrics that can handle all their ports without oversubscription, but often uplinks are still oversubscribed.
