Must an AES plain text to be encrypted be 128bit

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I am reading up on AES, but most say it takes a Plain text of 128bits so it can be used in a 44 matrix each of 1 byte where the basic operations like sub byte, shift row, mix column, would be performed on them. Must the plain text be 128bits? according to this website which allows one to run AES online. I used a plain text "big" and it still got encrypted. the text big cannot fill the 44 matrix, so what happens to the remaining space in the matrix ?.

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Chris Dodd On

If you use a 'block' mode (ECB or CBC) then the plaintext needs to be padded out to a multiple of the block size (128 bits), generally with 0 bits (though other schemes can be used)

If you use a 'stream' mode (CFB, OFB, or CTR), there's no need to pad out the input -- it can be any length (in bits) and the resulting ciphertext will be the same length.