That's Google's definition, which in my opinion doesn't capture the word correctly. That definition implies "plausible" suggests "likely". Turning to Webster:

    1 superficially fair, reasonable, or valuable but often specious <a plausible pretext>

    2 superficially pleasing or persuasive <a swindler… , then a quack, then a smooth, plausible gentleman — R. W. Emerson>

    3 appearing worthy of belief

To me, an idea is plausible if I can entertain the possibility without suspension of disbelief.