The road to Hell is paved with good intentions is a proverb. An alternative form is Hell is full of good meanings, but Heaven is full of good works.
One meaning of the phrase is that individuals may have the intention to undertake good actions but nevertheless fail to take action. This inaction may be due to procrastination, laziness or other subversive vice.
As such, the saying is an admonishment that a good intention is meaningless unless followed through.
A different interpretation of the saying is wrongdoings or evil actions are often masked by good intentions, or even that good intentions, when acted upon, may have unforeseen bad consequences.