Why Forward error correction adds redundant bits so the receiver can repair errors without a retransmission. The code rate is the ratio of useful data bits to total transmitted bits, so 3/4 means that for every 4 bits sent, 3 carry payload and 1 is added FEC overhead. One bit out of four is 25% of the transmission devoted to error correction. Lower code rates like 1/2 are more robust but carry less data in the same bandwidth.
Watch out The guard interval answer describes a different OFDM parameter, the fraction of each symbol period left blank to absorb multipath echoes, which is quoted separately as 1/4, 1/8, 1/16 and so on. Compression ratios are also specified independently of the coding rate.
Code rate = payload/total. 3/4 means 3 data bits per 4 sent, so the leftover 1 in 4, 25%, is FEC.