The libor swap rates show the fixed rate you would have to pay if you entered into a swap agreement where you received the floating 3-month libor rate.
From the link in your question:
Two Year: 0.478
Three Year: 0.549
Five Year: 0.842
For example, if I wanted to enter into a two year interest rate swap I would have to pay a fixed rate of 0.478 % for two years and in return I would receive interest payments based on the 3-month LIBOR rate (currently 0.4551 %). My interest payments would be fixed while the money I received from the swap would be variable based on the 3-month libor rate.
Mid-market means the rates were reported at the middle of the trading day.
Semi-annual means the swap settles interest payments every 6 months.