Voltage, Electrical Potential, Potential Difference, Electric Tension - whatever you refer to it as - is the always measured between two points.
Generally of course we mean two points within one circuit, and most of the time we mean between something and the circuit's 0-Volt rail; it's implied whenever somebody quotes a voltage, unless they specifiy otherwise.
We always refer to the 0-Volt rail as 'ground', no doubt stemming from the term 'earth', which is literally a connnection into the ground itself. In the case of a battery powered circuit, the 0-volt rail is only 0 Volts referenced to the rest of the circuit. In such a circumstance, the circuit's ground is said to be floating when referenced to another circuit. Thus, two circuits with independent power supplies and no interconnection cannot be compared to one another in sense of voltage.
This typically doesn't matter, but when you need to interconnect independently-powered circuits you run into a problem. As both grounds are floating, any other voltage on each circuit is referenced to said floating voltage, and therefore is also floating. This makes wired communication between them impossible.
Unless you create a voltage reference between the two.
Simply connect the ground rails of each together; the cicruits are then referenced to one another at 0 Volts. It doesn't have to be the ground rails that are connected - any two points connected together will allow you to reference any point on board A to board B and get a correct potential difference. However, it's pretty useless doing that except in some odd situations.