To explain it in a simple way:

-t rsa stands for the Type of algorithm, which is RSA, a bit outdated while widely supported.

-b 4096 is for the key length of 4096 bits, for better security, because the default is 2048.

Modern alternative is ed25519 algorithm, which is safer and faster with shorter keys.

Technology advancements, especially the quantum computing, lowers the strength of older cryptography algorithms, this is why newer stronger algorithms are being developed.

Older systems may not support newer algorithms, so longer keys (e.g. 4096 instead of 2048) is a way to strengthen the cryptographic protection with older algorithms.