As well as the definitions provided here, you can find some of these definitions in the Ethereum Glossary
BestBlock: The highest block number of the longest valid chain, said differently it is heavy with cumulative difficulty.
Uncles: Orphaned blocks and unlike other blockchains, in Ethereum they are included and rewarded. The dashboard displays current bloc uncle count and that from last 50 blocks.
LastBlock: The time since the last mined block in seconds.
AverageBlock: Average time between two block (excl. uncles) - should display 15 seconds unless the network is really slow.
AverageNetworkHashrate: Number of hashes spent by miners to find a new block. 250 TH/s is 250 trillion network hash power per second.
Difficulty: Also known as the mining difficulty to find a new block.
ActiveNodes: The number of nodes connected to the dashboard. The total number of active connected nodes powering the network is quite high.
GasPrice: The price miners are currently accepting. Currently it is ~40 gwei, which is 40 giga wei, which is 40 billion wei. The default is 20 gwei.
GasLimit: The gas limit for a block. The dashboard reads 7,999,992 gas and the miner can include transactions until the block is full and limit is met. The default is 4,712,388 (1.5 pi million gas) In bitcoin you might have come across block size limit.
PageLatency: A dashboard stat.
Uptime: A dashboard stat.
BlockTimeChart: Maps out actual time between blocks.
DifficultyChart: Maps out actual difficulty of previous blocks.
BlockPropagationChart: For the nodes connected to the dashboard, it is how fast blocks are shared/propagated between nodes.
LastBlockMiners: Displays the public key of the last miners who found the most number of blocks.
UncleCountChart: Displays number of uncles per 25 blocks per bar.
TransactionsChart: Displays the number of transactions included in previous blocks.
GasSpendingChart: Correlated to Transactions Chart and displays the amount of spent gas.
GasLimitChart: Dynamic in nature and shows Gas Limit for the previous blocks.