RSA
In plain terms
A foundational asymmetric (public-key) encryption algorithm.
Going deeper
RSA uses a public/private key pair for encryption and digital signatures, secured by the difficulty of factoring large numbers. It enabled secure key exchange and signing.
Under the hood
Slower than symmetric crypto, so used for key exchange/signatures; large keys needed, with ECC as a leaner alternative.
Examples
TLS handshakes, digital signatures, key exchange, SSH keys.
Think of it like...
Like a mailbox anyone can drop letters into (public key) but only you can open (private key).