The tax year (or financial year) is the period on which you are taxed by HMRC. In the UK, the tax year is 6th April to 5th April each year. In some instances, a business' financial year can be different.
This is usually due to the business starting to trade part way through a tax year and opting for their year-end to be one year from the date of commencement.
However most businesses will set their period end as the 5th April so that it falls in line with the tax year, as this is the more straight forward option.