Chain Tip
The most recent block at the front of the chain that nodes are building on.
The chain tip is the newest block in the heaviest valid chain - the block that miners are currently trying to build the next block on top of. When your node block height matches the chain tip reported by the wider network, your node is fully synced and up to date. Occasionally the network briefly has two candidate tips when two miners find a block at almost the same moment; nodes resolve this by following the branch that gets the next block, which becomes the new single tip. Watching the tip advance block by block is how you confirm your node is keeping pace with the live network rather than lagging behind.