Weak Randomness

Randomness is considered weak if it can be predicted in a non-negligible way. Broadly speaking, the true source of randomness is just a topic for discussion and yet to be proved, so any application that requires some should choose a certain level to maintain. In practice, most applications use default computer integrated randomness (like crypto.randomBytes() in NodeJS), which is sufficient.

Solidity

Smart contracts often need randomness to execute their logic fairly. Usually, there are several solutions:

• pseudorandomness from chain attributes
• decentralized services like Chainlink VRF
• commitment to an off-chain computation with later reveal, which often would require a centralized party
• other schemes

Each method has its strong sides. The most secure would be Chainlink VRF, but it will also require a small payment for each request.