These gauges indicate how fast the Bitcoin network is running.
Transactions per second is the large gauge on the left, this is the number of new transactions coming in averaged over a minute. If there are more than 3 Tx/s we are getting in the realm of too many. (theoretical 7 tx/s is only if every transaction is 240 bytes)
Top right, shows the number of unconfirmed transactions, currently in the Bitcoin memory pool. we can fit approx 4,000 average transactions in a block, so if we have more than that, then some transactions may have to wait before they are added to a block.
Finally bottom left shows how full the the last 6 blocks were on average. We currently have a hard limit of 1 megabyte per block on the Bitcoin network.