Lets start with what a concentration is. Concentration is the amount of something in a volume. For example, 1 gram of sugar in one litre of water is 1 gram per litre. This can be written as 1g/L (the division sign means "per"). When we are talking about moles, we are talking about an amount of something. 1 mole actually equals a certain amount of molecules (6.03x10^23 molecules to be exact). So one mole of sodium hydroxide literally means 6.03x10^23 molecules of sodium hydroxide.
Molar is a concentration. It means "moles per litre". So because mole is an actual amount, this means the amount of a molecule in a litre of liquid. For example, 1 molar sodium hydroxide would mean 1 mole of sodium hydroxide (6.03x10^23 molecules) in 1 litre.