Coin value is the money value of a single coin on slots that express bets in coins. It is one of the parts that make up the total bet, along with the number of coins per line (bet level) and the number of lines.
Some games show wins in coins rather than money, so knowing the coin value is how you convert them. Illustrative example: a win of 200 coins at a 5p coin value is £10.
The system comes from land-based machines that took physical coins of different denominations. Many current games hide it and show cash amounts throughout, which is easier to read. Coin value has no effect on odds or RTP; it only scales the amounts.