The Value of a commodity can be expressed in a certain quantity of some other Use Value. This quantity is the Equivalent Form of the commodity's Value. For example, the Value of 1 metre of linen can be expressed as 10 kilos of sugar. 10 kilos of sugar is the Equivalent Form of the Value of 1 metre of linen.
If this expression is turned around, we can express the Value of sugar as a certain quantity of the other Use Value, Linen. Then we would have 10 kilos of sugar expressed as 1 metre of linen. Then 1 metre of linen is the Equivalent Form of Value of 10 kilos of sugar. Exchange Value can only be expressed as a certain quantity of some other Use Value, whilst Value itself can be expressed as a certain quantity of Abstract Labour Time.
Where some commodity is separated off from other commodities, in order to act as an Equivalent Form of Value, against which all other commodities can be measured, this Universal Equivalent Form of Value can act as a money commodity.
For a fuller discussion see Capital I, Chapter 1 - Part 4