解码 is a separable verb-object compound (离合词) meaning 'to decode' or 'to decipher'. The verb 解 ('to untie/resolve') and the object 码 ('code') can be separated when you need to insert a measure word, result complement, or time duration (e.g., 解了两个码 'decoded two codes'). In everyday use, it often refers to decoding QR codes, barcodes, or encrypted messages.