I understand 12 word seeds mean there is 128 bits of 'security' but what does that mean? Would i need to worry about this being brute forced? If it was brute forced would having to scan the blockchain for every attempt not make that impossible/take forever? thanks guy
See here for considerations on security.
Note that it is only partially true that brute forcing would involve having to scan the blockchain for every attempt. If you're doing a random search to find some random address with a balance then yes, you need to scan the blockchan for each to know whether you've hit the jackpot. If you're doing a targeted brute-force against a known address (or a list of them) then - no. This is because just rolling the seeds until the resulting address matches is enough. To actually see the balance, you'd still have to scan, though.