"Associates" in this context means, for example, children, siblings, parents, grandparents, cousins and nephews/nieces.
When letting to associates (family), the same rules apply as for ordinary letting, as long as the letting takes place at the market price.
If the associate (family) gets to live in your property free of charge or at a reduced price, this will generally be a tax liable benefit for the family member in question. However, the so-called "associate rule" does allow family use of the house to be tax-free, under certain conditions.