To understand this, you first need to understand how an electric motor works.
An electric motor is, grossly oversimplified, a bunch of electric coils and magnets. There are different kinds of electric motors 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 helix. 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 all this electric energy is going when there is no kinetic energy being created. It gets converted into heat. The coils will heat up. Some motors are designed to withstand that heat buildup, but others are not. They will overheat, get damaged and might in the worst case even catch fire. So don't obstruct an electric motor, unless you know it's designed for that.