Think of it as a road.
I'll give you an example:
Lets say you have a 1 lane road. The represents a 1 core CPU.
The 1 core CPU wil have "traffic" since it's only 1 core, but you're putting a lot of stress on it. This will spend more time doing a process.
Now lets say you have a 2 lane road. This road will have less traffic, because each core is able to organize the work, making less stress, and less time spent to finish.