The validation block is meant to validate your data in your flow design. This can be data inputted from the user, or data captured from an external system. The validation works like a condition block, but without multiple routes. This helps to simplify your flow and give an easier overview.
If the validation fails you have the following options:
repeat previous block
ask for variable (let user (re)input variable)
send a message (give input block)
skip (next/continue)
restart flow
redirect to flow