This saying means that it is never right, and always wrong, for a Christian to hate any person - and even when that person is (or is regarded as) evil, this does not make any difference.
Hatred is a sin and therefore must be repented.
It also means that it is okay for a Christian to love any person, no matter how evil his behaviour.
This is the opposite of the prevailing secular mainstream ethic, which is to Love everybody equally in an abstract and impersonal sense; but to engage in unrestrained hatred towards specific persons who behave in a way that is (or is regarded as) evil; this modern ethic also prohibits its adherents from loving any specific person whose behaviour is (or is regarded as) evil.
So for Christians, hating the sin does not mean hating the sinner - but prohibits this; loving the sinner does not mean condoning, tolerating, approving, encouraging, promoting or legally (or by other means) enforcing his sin.
It probably also needs pointing-out that 'loving the sinner' is compatible with - and indeed has often entailed - resisting him, warning others against him, fighting him, and even at times killing him.