Interlocking is usually mechanical or relay based. In such interlocking systems, the various routes and switches outside a station are physically connected together so that setting one route through the station automatically disables all other routes.
For example, in a station with 2 loop lines and a main line, if a train is going to pass through the main line without stopping, the main line signal will be set to green and the switches will be set so that the train stays on the main line. In a mechanical interlocking, the points and signals will be connected in such a way that turning the signal green automatically prevents the points from being changed until the signal is...
more... changed to Red. This prevents accidental changing of points while the train is passing through. Relay based systems also do a similar thing where the electronic switches operating the points are connected in such a way that switching on a few switches automatically disconnects other switches.
In these systems, if the routes in the interlocking have to be changed, say, after construction of a new loop line, then all the mechanical/electrical connections have to be relaid again. In a Solid state interlocking, all the switches controlling the points and signals are controlled via software running on microprocessors ( Hence the name 'solid state', which is the general term used for semiconductor devices like microprocessors). In these systems, changing the routes can be done purely through software changes and does not require any physical modifications in the Yard.