Taxes are computed a little differently than traditional accounting books.
Tax authorities are more interested in when cash enters and leaves a company, while most accounting books are concerned with accrurals.
Sometimes there's a bit of confusion when differences between the two show up.
Sometimes a company receives cash, but hasn't earned it yet. This happens every time there is unearned revenue.
Sometimes a company completes a job, but hasn't yet received cash.
This is where deferred tax assets and deferred tax liabilities come in.