what does the .sat / byte value mean?
Satoshi is the smallest unit of BTC - it's a 100,000,000th of one Bitcoin. Bytes are the unit that Electrum is using to measure how much space your transaction is expected to take up on the blockchain. So satoshi per byte is just a measurement of what fees you're paying relative to how much space your transaction is going to take up on the blockchain.
What are the drawbacks of setting such low taxes? slow down the transactions?
Yes.
how much?
It varies from time to time, but you could look at
this site for estimates of how long your transaction is going to take to confirm with different fees with the current network conditions.
Generally, you can send a fairly low fee if you're willing to wait for a while.