The paradox of thrift is a pretty basic concept described in the video as being the idea that saving hurts the economy because it keeps other businesses from having that money but when you spend it keeps that economy running and flowing making you richer.
Critique of POT: If everyone embraced this idea, people would ultimately not have as much money to spend because they would not be making interest, which would also lead to lower aggregate demand. Excessive spending also increases the amount of debt people are in, which is bad for the individual.
This might be true if everyone was saving their money at their house. If people put their money in the bank, other people can then take out loans which is actually better for the economy.