Okay, too much ego. Generally when we think of too much ego in the normal sense of the word, it is a person who is kind of arrogant, they think big of themselves. Maybe some of it is merited, some of it is not merited and they have a puffed up image of themselves that they are better than others. May be they have rubbed us up the wrong way and we don't like being around them. That is how we would normally think of someone who has too much ego.
In Buddha/Dhamma terms too much ego refers to a big spiritual ego. They have done so many retreats and they make sure they tell you all about that. They have practiced so many years and they make sure they tell you that. They tell you about some of their concentration phenomena they have had in their meditation, how blissful it was, they tell you how long they can sit, they tell you all these different things, they may also tell you all the teachers they know and their names, etc., etc. They tell you all their statistics. That would be a spiritual person with too much spiritual ego. So too much ego in Buddha/Dhamma terms refers to a person that puffs themselves up by telling you all about their meditation experience and statistics.
Now how does this hinder development on the path? It is spiritual suicide. It is a person who gets stuck, it is a person who is content with where they are at, in fact most don't even know where they are at because they are so full of their ego, so full of themselves. They are not trying to let go of Dukkha, they are creating more Dukkha for themselves. So their practice will start going backwards, if they are not already going backwards. Having too much ego is certainly something that will hinder personal development and take their practice downhill.