Confirmations
A confirmation is each new block added after the block that contains your transaction.
In depth
One confirmation means your transaction is in the latest block; six means five more blocks have been built on top of it. Each additional block makes reversing the transaction exponentially harder. Exchanges wait for a set number of confirmations before crediting deposits, with the number depending on the chain and amount.
Why it matters
It is why a deposit can be "visible" yet "pending" — the network is still cementing it.