A valid Monero mnemonic seed is a phrase with 13, 14, or 25 random words (multiple languages are supported). Your mnemonic seed has all the information needed to restore your wallet.
Most Monero wallets ask the user to write down a mnemonic seed on a paper backup when a wallet is being created. So it's up to you to remember where and how you stored your mnemonic seed.
Alternatively, if you still have access to the wallet being restored, you can usually find your mnemonic seed in a menu with some name like `Seed`, `Keys`, `Secrets`, or `Master Private Key`.
The wallet restore height is a setting that tells your wallet software at which block height it should start scanning the blockchain looking for transactions. Ideally it should be the number of the block which included the first incoming transaction to your wallet.
If you set a restore height higher than the block height of your first incoming transaction, your wallet software will not detect all transactions of your wallet. In consequence, your balance will be wrong and your wallet software will not work properly.
If you set a restore height lower than the block height of your first incoming transaction, your wallet software will scan unnecessary blocks. In consequence, your initial wallet synchronization will take longer than necessary, but your wallet software will still work.
Therefore, if you're unsure about the block height of your first incoming transaction, it's always better to set a lower restore height instead of a higher one.
