To understand this, you first need to understand how an electric motor is working.
An electric motor is, grossly oversimplified, a bunch of electric coils and magnets. There are different kinds of electromotors with different arrangements of coils and magnets. Sometimes the magnets are permanent-magnets, sometimes they are electromagnets, also made from coils. But in the end, the parts which draw current are the coils.
A coil is an electric wire wrapped in a spiral. When you put a voltage on the ends of a wire, the wire draws a current. When there is a current in a wire, a magnetic field is created around it. By wrapping a wire in a spiral, the magnetic fields of the individual windings add up and create a linear magnetic field along the coil. Interacting magnetic fields create motion. And that's how a motor moves.
When the motor can not move for some reason, like when it is obstructed somehow, this is no reason why the wires which make up the coils should suddenly increase their resistance. They still allow current to go through them. You might wonder where this electic energy is going when there is no kinetic energy being created. You can expect it to get converted into heat mostly. The coils will heat up. This might even damage some motors in the long run, so make sure not to obstruct the movement of an electric motor for too long (at least not without consulting the manual).