If something is ineffable, it is too great, too beautiful, too holy, or too strange to put into words. You feel it clearly, but language is not enough. This is a literary, rather churchy or poetic word, not everyday chat. Writers use it for God, for a huge feeling of wonder, for music, for love, or for a moment that stops you. Though actually, people also use it more loosely now to mean 'amazingly good' or 'I cannot explain how strong this feeling is', even when they could find words if they tried. It is not the same as 'I do not know the word'. The point is that words fail because the thing itself is beyond them.