It's used to treat anxiety, muscle spasms and fits (seizures). It's also used in hospital to reduce alcohol withdrawal symptoms, such as sweating or difficulty sleeping.
If you're taking diazepam regularly and forget to take a dose, take the missed dose as soon as you remember, unless it's nearly time for your next dose.
In this case, just leave out the missed dose and take your next dose as normal.
Never take 2 doses at the same time. Never take an extra dose to make up for a forgotten one.