Plaintext can refer to anything which humans can understand and/or relate to. This may be as simple as English sentences, a script, or Java code. If you can make sense of what is written, then it is in plaintext.
Ciphertext, or encrypted text, is a series of randomized letters and numbers which humans cannot make any sense of. An encryption algorithm takes in a plaintext message, runs the algorithm on the plaintext, and produces a ciphertext. The ciphertext can be reversed through the process of decryption, to produce the original plaintext.
Example: We will encrypt a sentence using Caesar Cipher. The key is 7, which means the letter a becomes h.
Plaintext: This is a plaintext.
Ciphertext: Aopz pz h wshpualea.