Single-sided liquidity is a position that holds only one of the two tokens in a pool.
Single-sided liquidity is only available when using the Uniswap v3 protocol. This allows you to provide liquidity with a single token of the pair.
The pools only hold both tokens in the current liquidity range. This is the range where all swaps are completed.
When supplying liquidity in Uniswap v3 you are able to choose your range. When you select a range that is outside the current price range, you will only be able to supply one of the two tokens.
Here is an example of what the price range will look like if it is above the current price.
Here is an example of what the price range will look like if it is below the current price.