I can understand being deceived into thinking it's a joke. subject - I verb - can (modal) understand (verb) object (gerund phrase) - being deceived into thinking it's a joke. gerund - being deceived gerund complement - into thinking it's a joke preposition - into gerund #2 - thinking gerund complement #2 - [that] it's a joke

The most complete meaning for the original sentence would be: I can understand how someone could be deceived into thinking it's a joke. The use of 'deceived' here is awkward (but grammatically it is correct). A more natural version: I can understand how someone could be tricked into thinking it's a joke. In English, when you use a gerund (-ing) form of a verb in a sentence like this, there is an implied empathy with any subject of the verb, with the adverb 'how' or 'why.' Examples: I can understand running every day to stay healthy. (I can understand why someone would run every day to stay healthy.) I can understand being fooled into buying cheap goods from an online store. (I can understand how someone could be fooled into buying cheap goods....) I can understand being knocked down by a hurricane-level wind. (I can understand how someone could be knocked down by a hurricane-level wind). etc. Hope this helps.

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