As an adjective, 'flat' can mean 'level' - something that doesn't go up or down.

'This table is flat.'

We can also use 'flat' as adverb. It refers

to something which is lying horizontally.

'This book is lying horizontally!' Or: 'It's lying flat.'

When we feel sad, or dull, or we lack emotion,

we can say that we 'feel flat'.

'I feel flat because I can't go outside today!'

As a noun, a 'flat' is also a place we live. We could also say 'apartment'.

'I live in a flat in London.'

We can also use 'flat' after a period of time

to emphasise how quickly something can be done.

'This is a short video. You can watch it in one minute flat.'