or

What are lanthanides and act actinides? Why are they kept or placed at the bottom of the periodic table?

The 14 elements from cerium te lawrencium after lutetium are called lanthanides.
The elements from thorium to lawrencium after actinium are called actinides.

These elements are placed at the bottom of the periodic table because of their similar properties. Lanthanides are rare metals whereas actinides are artificial radioactive metals. They show similar properties to the transitional elements. So, they are placed separately below the transitional elements in blocks of the modem periodic table.