There has always been a degree of uncertainty around whether or not some people are resident in the UK for tax purposes. The recent changes to the rules are meant
to improve this situation. Why is this important? Well if you are resident for tax purposes then you are liabel on worldwide income and capital gains.
If you are not resident then foreign income and gains should usually escape the UK tax man.

This is a very brief simplification of the rules

You are non resident for a tax year if

1) You spend less than 16 days in the UK; or
2) You were non-resident in all of the previous 3 tax years and this year you spend less than 46 days in the UK; or
3) You are working full time abroad.

If none of the three points above apply then you will be resident in the UK if either

1) You spend 183 days or more in the UK; or
2) You have your only or main home in the UK; or
3) You work full time in the UK.

If none of the above six tests apply then we look at a combination of the number of days spent in the UK along with the strength of your ties to the UK.The more
ties you have then the fewer days you can spend here if you want to escape the UK tax regime for foreign earnings and gains.