Plausible deniability may refer $1)$ to deniable encryption schemes allowing the decryption of a ciphertext for a message $m$ to some distinct message $m'$ or $2)$ to a feature provided by deniable file systems.
Plausible deniability achieved by means of deniable encryption means that if a person who has encrypted some message $m$ to a ciphertext $c$ is forced (by an attacker) to provide the key, then this person can provide a fake key (or random choices) that can be used to decrypt the ciphertext to a different message $m'$. If for the attacker it is convincing that the ciphertext is an encryption of message $m'$, the encryption scheme is called deniable and plausible deniability is achieved.
Plausible deniability also refers to a feature provided by deniable file systems, meaning that one can plausibly deny the existence of files. This is typically achieved by means of deniable encryption.