If someone happens to get my seed phrase (off a paper I wrote it on) can they restore my Daedalus wallet on another machine and take my ADA?
The seed phrase is used to generate your private keys (and thus your public keys as well). If someone has access to your seed phrase, they can "restore" your wallet Yoroi, Daedalus, or any other wallet application and can do anything with your wallet that you could do. The only way to know if this has happened is to notice any transaction sending Ada or a token from your wallet that was not done by you. If you have any doubt, just create a new wallet and transfer all of your tokens and then funds from your old potentially compromised wallet to your new wallet.
As others have stated, yes they can restore your wallet and yes they can take all your ada and anything else associated with it.
With that said, here are some tips for securing your mnemonic phrase.
1.) Do not store these phrases on a computer. And if you must, do it on a air-gapped machine.
2.) Make sure you protect yourself from natural disasters. For example what if your house burns down, burning your computer with the restored wallet as well as the piece of paper the mnemonic was on? I personally stamped mine into a metal plate and keep it in a fireproof safe.
3.) Make sure you include instructions in your safe for a what to do with it in case you die. Personally my family does not know much about crypto so I would want my beneficiaries to have instructions on what to do. This way a loved one gets those gains in case something were to happen to you.
4.) Bonus tip, get a hardware Wallet like a Ledger X. This insures that your keys are not compromised in the making of your wallet. Although probably unlikely, it is possible that your internet connected device, you make the wallet on, is compromised. If it is, there is a possibility your wallet is too.
In Hierarchical Deterministic (HD) Wallet the seed phrase is used to generate a serie of public (and its correponding addresses) and private keys pairs (accounts).
A specific seed will always generate the same set of pairs.
If someone accesses your mnemonic, he will be able to generate the private key for the account containing the funds (usually the first account derived). With the private key, he will be able to sign a transaction and steal the funds.
In addition to securing your seed phrase using the above methods, you may also want to memorise it. You can do that by grouping it sets of maybe four and creating silly sentences each containing four of the words. Every three days, write down the sentenses while highlighting the keywords. Do that for some months and it sticks.
This way, if I ever need to move my funds in an emergency (like suspecting someone may have had access to the physical backup), I can move fast by downloading a wallet anywhere and importing the wallet from my memorised seed phrase and moving the funds.