Specific impulse is a measure of the efficiency of a reaction-drive engine (e.g. a rocket). It measures the amount of momentum change that is delivered for the expenditure of a set mass of propellant.
For example, the Delta-glider's main engines have a specific impulse of 40 kN·s/kg. For each kg of propellant expended, they will produce 40kN of thrust for one second - or one kN of thrust for 40 seconds.
The formula N·s/kg can be reduced to m/s; specific impulse could be considered to represent the exhaust velocity of an idealised rocket.
Note that American usage treats specific impulse somewhat differently; the use of pounds-thrust as well as pounds-mass leads to a figure in (lbthr·s/lb), with the lbthr and lb being naively cancelled to give a number expressed as a time. See for example this NASA page which describes a rocket of having a specific impulse of "240 seconds". To convert this to a proper formulation, remember that this should really be "240 gravity seconds" - the Earth's surface gravity is a hidden factor. Multiply 240s by 9.81 m/s/s and you get the true value, 2354.4 m/s. Of course, if you're just comparing different spacedrives rather than doing detailed calculations, this doesn't matter - just make sure all the specific impulse values you use follow the same convention.
The maximum thrust of a drive is unrelated to its specific impulse.
The total Delta_V of a ship is , where F is the fuel mass, M is the dry mass, and Isp is the specific impulse of the drive.
See also Wikipedia: